(Ada) fix handling of minimal symbols (UNOP_CAST and UNOP_ADDR)

Consider a program which provides a symbol without debugging
information. For instance, compiling the following code without -g:

    Some_Minimal_Symbol : Integer := 1234;
    pragma Export (C, Some_Minimal_Symbol, "some_minsym");

Trying to print this variable with GDB now causes an error, which
is now expected:

    (gdb) p some_minsym
    'some_minsym' has unknown type; cast it to its declared type

However, trying to cast this symbol, or to take its address
does not work:

    (gdb) p integer(some_minsym)
    'some_minsym' has unknown type; cast it to its declared type
    (gdb) p &some_minsym
    'some_minsym' has unknown type; cast it to its declared type

Another manisfestation of this issue can be seen when trying to
insert an Ada exception catchpoint for a specific standard exception
(this only occurs if the Ada runtime is built without debugging
information, which is the default).  For instance:

    $ (gdb) catch exception constraint_error
    warning: failed to reevaluate internal exception condition for catchpoint 0: 'constraint_error' has unknown type; cast it to its declared type

This is because, internally, the cachtpoint uses a condition referencing
a minimal symbol, more precisely:

   long_integer (e) = long_integer (&constraint_error)

This patch fixes all issues listed above:

  1. resolve_subexp: Special-case the handling of OP_VAR_MSYM_VALUE
     expression elements, where there are no ambiguities to be resolved
     in that situation;

  2. ada_evaluate_subexp: Enhance the handling of the UNOP_CAST
     handling so as to process the case where the target of
     the cast is a minimal symbol (as well as a symbol with debugging
     information). This mimics what's done in C.

gdb/ChangeLog:

        * ada-lang.c (resolve_subexp): Add handling of OP_VAR_MSYM_VALUE.
        (ada_evaluate_subexp_for_cast): New function.
        (ada_evaluate_subexp) <UNOP_CAST>: Replace code by call to
        ada_evaluate_subexp_for_cast.
        (ada_evaluate_subexp) <nosideret>: Replace code by call to
        eval_skip_value.
        * eval.c (evaluate_var_value): Make non-static.
        (evaluate_var_msym_value, eval_skip_value): Likewise.
        * value.h (evaluate_var_value, evaluate_var_msym_value)
        (eval_skip_value): Declare.

gdb/testsuite/ChangeLog:

        * gdb.ada/minsyms: New testcase.

Tested on x86_64-linux. No regression. Fixes the following failures:

    catch_ex.exp: continuing to Program_Error exception
    catch_ex.exp: continuing to failed assertion
    catch_ex.exp: continuing to unhandled exception
    catch_ex.exp: continuing to program completion
    complete.exp: p <Exported_Capitalized>
    complete.exp: p Exported_Capitalized
    complete.exp: p exported_capitalized
    mi_catch_ex.exp: catch Program_Error (unexpected output)
    mi_catch_ex.exp: continue to exception catchpoint hit (unknown output after running)
    mi_catch_ex.exp: continue to assert failure catchpoint hit (unknown output after running)
    mi_catch_ex.exp: continue to unhandled exception catchpoint hit (unknown output after running)
    mi_ex_cond.exp: catch C_E if i = 2 (unexpected output)

1 files changed, 21 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.ada/minsyms/pck.ads b/gdb/testsuite/gdb.ada/minsyms/pck.ads
new file mode 100644
index 0000000..4ad4c07
--- /dev/null
+++ b/gdb/testsuite/gdb.ada/minsyms/pck.ads
@@ -0,0 +1,21 @@
+--  Copyright 2017 Free Software Foundation, Inc.
+--
+--  This program is free software; you can redistribute it and/or modify
+--  it under the terms of the GNU General Public License as published by
+--  the Free Software Foundation; either version 3 of the License, or
+--  (at your option) any later version.
+--
+--  This program is distributed in the hope that it will be useful,
+--  but WITHOUT ANY WARRANTY; without even the implied warranty of
+--  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+--  GNU General Public License for more details.
+--
+--  You should have received a copy of the GNU General Public License
+--  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package Pck is
+   Some_Minimal_Symbol : Integer := 1234;
+   pragma Export (C, Some_Minimal_Symbol, "some_minsym");
+
+   procedure Increment (I: in out Integer);
+end Pck;