[gdb/ada] Handle artificial local symbols

[gdb/ada] Handle artificial local symbols

With current master and gcc 7.5.0/8.5.0, we have this timeout:
...
(gdb) print s^M
Multiple matches for s^M
[0] cancel^M
[1] s at src/gdb/testsuite/gdb.ada/interface/foo.adb:20^M
[2] s at src/gdb/testsuite/gdb.ada/interface/foo.adb:?^M
> FAIL: gdb.ada/interface.exp: print s (timeout)
...

[ The FAIL doesn't reproduce with gcc 9.3.1.  This difference in
behaviour bisects to gcc commit d70ba0c10de.

The FAIL with earlier gcc bisects to gdb commit ba8694b650b. ]

The FAIL is caused by gcc generating this debug info describing a named
artificial variable:
...
 <2><1204>: Abbrev Number: 31 (DW_TAG_variable)
    <1205>   DW_AT_name        : s.14
    <1209>   DW_AT_type        : <0x1213>
    <120d>   DW_AT_artificial  : 1
    <120d>   DW_AT_location    : 5 byte block: 91 e0 7d 23 18   \
      (DW_OP_fbreg: -288; DW_OP_plus_uconst: 24)
...

An easy way to fix this would be to simply not put named artificial variables
into the symbol table.  However, that causes regressions for Ada.  It relies
on being able to get the value from such variables, using a named reference.

Fix this instead by marking the symbol as artificial, and:
- ignoring such symbols in ada_resolve_variable, which fixes the FAIL
- ignoring such ada symbols in do_print_variable_and_value, which prevents
  them from showing up in "info locals"

Note that a fix for the latter was submitted here (
https://sourceware.org/pipermail/gdb-patches/2008-January/054994.html ), and
this patch borrows from it.

Tested on x86_64-linux.

Co-Authored-By: Joel Brobecker  <brobecker@adacore.com>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28180