Eliminate most remaining cleanups under gdb/guile/

The main complication with the Guile code is that we have two types of
exceptions to consider.  GDB/C++ exceptions, and Guile/SJLJ
exceptions.  Code that is facing the Guile interpreter must not throw
GDB exceptions, instead Scheme exceptions must be thrown.  Also,
because Guile exceptions are SJLJ based, Guile-facing code must not
use local objects with dtors, unless wrapped in a scope with a
TRY/CATCH, because the dtors won't otherwise be run when a Guile
exceptions is thrown.

This commit adds a new gdbscm_wrap wrapper function than encapsulates
a pattern I noticed in many of the functions using
GDBSCM_HANDLE_GDB_EXCEPTION_WITH_CLEANUPS.  The wrapper is written
such that you can pass either a lambda to it, or a function plus a
variable number of forwarded args.  I used a lambda when its body
would be reasonably short, and a separate function in the larger
cases.

This also convers a few functions that were using
GDBSCM_HANDLE_GDB_EXCEPTION to use gdbscm_wrap too because they
followed a similar pattern.

A few cases of make_cleanup calls are replaced with explicit xfree
calls.  The make_cleanup/do_cleanups calls in those cases are
pointless, because do_cleanups won't be called when a Scheme exception
is thrown.

We also have a couple cases of Guile-facing code using RAII-type
objects to manage memory, but those are incorrect, exactly because
their dtor won't be called if a Guile exception is thrown.

gdb/ChangeLog:
2018-07-18  Pedro Alves  <palves@redhat.com>

	* guile/guile-internal.h: Add comment about mixing GDB and Scheme
	exceptions.
	(GDBSCM_HANDLE_GDB_EXCEPTION_WITH_CLEANUPS): Delete.
	(gdbscm_wrap): New.
	* guile/scm-frame.c (gdbscm_frame_read_register): Use xfree
	directly instead of a cleanup.
	* guile/scm-math.c (vlscm_unop_gdbthrow): New, factored out from ...
	(vlscm_unop): ... this.  Reimplement using gdbscm_wrap.
	(vlscm_binop_gdbthrow): New, factored out from ...
	(vlscm_binop): ... this.  Reimplement using gdbscm_wrap.
	(vlscm_rich_compare): Use gdbscm_wrap.
	* guile/scm-symbol.c (gdbscm_lookup_symbol): Use xfree directly
	instead of a cleanup.
	(gdbscm_lookup_global_symbol): Use xfree directly instead of a
	cleanup.
	* guile/scm-type.c (gdbscm_type_field, gdbscm_type_has_field_p):
	Use xfree directly instead of a cleanup.
	* guile/scm-value.c (gdbscm_make_value, gdbscm_make_lazy_value):
	Adjust to use gdbscm_wrap and scoped_value_mark.
	(gdbscm_value_optimized_out_p): Adjust to use gdbscm_wrap.
	(gdbscm_value_address, gdbscm_value_dereference)
	(gdbscm_value_referenced_value): Adjust to use gdbscm_wrap and
	scoped_value_mark.
	(gdbscm_value_dynamic_type): Use scoped_value_mark.
	(vlscm_do_cast, gdbscm_value_field): Adjust to use gdbscm_wrap and
	scoped_value_mark.
	(gdbscm_value_subscript, gdbscm_value_call): Adjust to use
	gdbscm_wrap and scoped_value_mark.
	(gdbscm_value_to_string): Use xfree directly instead of a
	cleanup.  Move 'buffer' unique_ptr to TRY scope.
	(gdbscm_value_to_lazy_string): Use xfree directly instead of a
	cleanup.  Move 'buffer' unique_ptr to TRY scope.  Use
	scoped_value_mark.
	(gdbscm_value_fetch_lazy_x): Use gdbscm_wrap.
	(gdbscm_parse_and_eval): Adjust to use gdbscm_wrap and
	scoped_value_mark.
	(gdbscm_history_ref, gdbscm_history_append_x): Adjust to use
	gdbscm_wrap.

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