Target FP: Use target format throughout expression parsing

When parsing floating-point literals, the language parsers currently
use parse_float or some equivalent routine to parse the input string
into a DOUBLEST, which is then stored within a OP_DOUBLE expression
node.  When evaluating the expression, the OP_DOUBLE is finally
converted into a value in target format.

On the other hand, *decimal* floating-point literals are parsed
directly into target format and stored that way in a OP_DECFLOAT
expression node.  In order to eliminate the DOUBLEST, this patch
therefore unifies the handling of binary and decimal floating-
point literals and stores them both in target format within a
new OP_FLOAT expression node, replacing both OP_DOUBLE and
OP_DECFLOAT.

In order to store literals in target format, the parse_float
routine needs to know the type of the literal.  All parsers
therefore need to be changed to determine the appropriate type
(e.g. by detecting suffixes) *before* calling parse_float,
instead of after it as today.  However, this change is mostly
straightforward -- again, this is already done for decimal FP
today.

The core of the literal parsing is moved into a new routine
floatformat_from_string, mirroring floatformat_to_string.
The parse_float routine now calls either floatformat_from_string
or decimal_from_sting, allowing it to handle any type of FP
literal.

All language parsers need to be updated.  Some notes on
specific changes to the various languages:

- C: Decimal FP is now handled in parse_float, and no longer
  needs to be handled specially.

- D: Straightforward.

- Fortran: Still used a hard-coded "atof", also replaced by
  parse_float now.  Continues to always use builtin_real_s8
  as the type of literal, even though this is probably wrong.

- Go: This used to handle "f" and "l" suffixes, even though
  the Go language actually doesn't support those.  I kept this
  support for now -- maybe revisit later.  Note the the GDB
  test suite for some reason actually *verifies* that GDB supports
  those unsupported suffixes ...

- Pascal: Likewise -- this handles suffixes that are not
  supported in the language standard.

- Modula-2: Like Fortran, used to use "atof".

- Rust: Mostly straightforward, except for a unit-testing hitch.
  The code use to set a special "unit_testing" flag which would
  cause "rust_type" to always return NULL.  This makes it not
  possible to encode a literal into target format (which type?).
  The reason for this flag appears to have been that during
  unit testing, there is no "rust_parser" context set up, which
  means no "gdbarch" is available to use its types.  To fix this,
  I removed the unit_testing flag, and instead simply just set up
  a dummy rust_parser context during unit testing.

- Ada: This used to check sizeof (DOUBLEST) to determine which
  type to use for floating-point literal.  This seems questionable
  to begin with (since DOUBLEST is quite unrelated to target formats),
  and in any case we need to get rid of DOUBLEST.  I'm now simply
  always using the largest type (builtin_long_double).

gdb/ChangeLog:
2017-10-25  Ulrich Weigand  <uweigand@de.ibm.com>

	* doublest.c (floatformat_from_string): New function.
	* doublest.h (floatformat_from_string): Add prototype.

	* std-operator.def (OP_DOUBLE, OP_DECFLOAT): Remove, replace by ...
	(OP_FLOAT): ... this.
	* expression.h: Do not include "doublest.h".
	(union exp_element): Replace doubleconst and decfloatconst by
	new element floatconst.
	* ada-lang.c (resolve_subexp): Handle OP_FLOAT instead of OP_DOUBLE.
	(ada_evaluate_subexp): Likewise.
	* eval.c (evaluate_subexp_standard): Handle OP_FLOAT instead of
	OP_DOUBLE and OP_DECFLOAT.
	* expprint.c (print_subexp_standard): Likewise.
	(dump_subexp_body_standard): Likewise.
	* breakpoint.c (watchpoint_exp_is_const): Likewise.

	* parse.c: Include "dfp.h".
	(write_exp_elt_dblcst, write_exp_elt_decfloatcst): Remove.
	(write_exp_elt_floatcst): New function.
	(operator_length_standard): Handle OP_FLOAT instead of OP_DOUBLE
	and OP_DECFLOAT.
	(operator_check_standard): Likewise.
	(parse_float): Do not accept suffix.  Take type as input.  Return bool.
	Return target format buffer instead of host DOUBLEST.
	Use floatformat_from_string and decimal_from_string to parse
	either binary or decimal floating-point types.
	(parse_c_float): Remove.
	* parser-defs.h: Do not include "doublest.h".
	(write_exp_elt_dblcst, write_exp_elt_decfloatcst): Remove.
	(write_exp_elt_floatcst): Add prototype.
	(parse_float): Update prototype.
	(parse_c_float): Remove.

	* c-exp.y: Do not include "dfp.h".
	(typed_val_float): Use byte buffer instead of DOUBLEST.
	(typed_val_decfloat): Remove.
	(DECFLOAT): Remove.
	(FLOAT): Use OP_FLOAT and write_exp_elt_floatcst.
	(parse_number): Update to new parse_float interface.
	Parse suffixes and determine type before calling parse_float.
	Handle decimal and binary FP types the same way.

	* d-exp.y (typed_val_float): Use byte buffer instead of DOUBLEST.
	(FLOAT_LITERAL): Use OP_FLOAT and write_exp_elt_floatcst.
	(parse_number): Update to new parse_float interface.
	Parse suffixes and determine type before calling parse_float.

	* f-exp.y: Replace dval by typed_val_float.
	(FLOAT): Use OP_FLOAT and write_exp_elt_floatcst.
	(parse_number): Use parse_float instead of atof.

	* go-exp.y (typed_val_float): Use byte buffer instead of DOUBLEST.
	(parse_go_float): Remove.
	(FLOAT): Use OP_FLOAT and write_exp_elt_floatcst.
	(parse_number): Call parse_float instead of parse_go_float.
	Parse suffixes and determine type before calling parse_float.

	* p-exp.y (typed_val_float): Use byte buffer instead of DOUBLEST.
	(FLOAT): Use OP_FLOAT and write_exp_elt_floatcst.
	(parse_number): Update to new parse_float interface.
	Parse suffixes and determine type before calling parse_float.

	* m2-exp.y: Replace dval by byte buffer val.
	(FLOAT): Use OP_FLOAT and write_exp_elt_floatcst.
	(parse_number): Call parse_float instead of atof.

	* rust-exp.y (typed_val_float): Use byte buffer instead of DOUBLEST.
	(lex_number): Call parse_float instead of strtod.
	(ast_dliteral): Use OP_FLOAT instead of OP_DOUBLE.
	(convert_ast_to_expression): Handle OP_FLOAT instead of OP_DOUBLE.
	Use write_exp_elt_floatcst.
	(unit_testing): Remove static variable.
	(rust_type): Do not check unit_testing.
	(rust_lex_tests): Do not set uint_testing.  Set up dummy rust_parser.

	* ada-exp.y (type_float, type_double): Remove.
	(typed_val_float): Use byte buffer instead of DOUBLEST.
	(FLOAT): Use OP_FLOAT and write_exp_elt_floatcst.
	* ada-lex.l (processReal): Use parse_float instead of sscanf.

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