gdb: add annotation in 'info locals' command for variables shadowing case

For C/C++/Fortran/Ada languages GDB prints same name variable multiple
times in case of variable shadowing and it is confusing for user to identify
which variable belongs to the current scope.  So for such cases add location
info to the innermost listed variables and for super block variables add
"shadowed" annotation in the form of "<file.c:line, shadowed>".

Suppose we have

1:int x = 3;
2:  {
3:    int x = 4;
4:    int y = 52;
5:    x = 99; /* break here */
6:  }

Currently:

(gdb) info locals
x = 4
y = 52
x = 3

After applying this patch, we obtain:

(gdb) info locals
x = 4  <file.c:3>
y = 52
x = 3  <file.c:1, shadowed>

The patch adds the location annotations by keeping track of inner block
and already printed variables to identify shadowing.  So, GDB now prints
"<file.c:line, shadowed>" for shadowed super-block variables and
"<file.c:line>" for innermost declarations of such variables only.

The location annotations are printed for shadowed variables in case of
C/C++/Fortran/Ada languages.  In Rust, it is possible to declare a
variable with the same name many times.  So in this case, just the first
instance of the variable is printed.  RUST language test "var_reuse.exp"
fails with rustc compiler version >= 1.73 so XFAIL is added accordingly.

Fix regex expression in the gdb.opt/inline-locals.exp test according to
this change.  The test update is only required due to the existing gdb
known ticket gdb/25695 where this issue is seen with 7.5.0 version on
sles15sp6 but it is not seen anymore on the newer gcc versions e.g.
gcc-11.4.0.

The symtab()/filename() nullptr check was added specifically to avoid
the crash seen in gdb.dwarf2/missing-type-name-for-templates.exp where
template symbols may have no associated source file.

Reviewed-By: Guinevere Larsen <guinevere@redhat.com>
Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Co-Authored-By: Andrew Burgess <aburgess@redhat.com>
Approved-By: Andrew Burgess <aburgess@redhat.com>