New Ada maintenance command to ignore descriptive types (DWARF).

Currently, Ada debugging requires the use of certain GNAT-specific
encodings, which are generated by the compiler. These encodings
were created a long time ago to work around the fairly limited
capabilities of the stabs debugging format. With DWARF, the vast
majority of the encodings could be abandoned in favor of a pure
DWARF approach.

In order to make it easier to evaluate the quality of the DWARF
debugging information generated by the compiler, and how the debugger
handles it, we are introducing a small Ada-specific maintenance
setting which changes the debugger's behavior to ignore descriptive
types. Descriptive types are artificial types generated by the
compiler purely to give the debugger hints as to how to properly
decode certain properties of a type.  For instance, for array
types, it generates a parallel type whose name is the name of
the array suffixed with ___XA, whose contents tells us what
the array's index type is, and possibly its bounds. See GCC's
gcc/ada/exp_dbug.ads for the full description of all encodings.

This is only a first step, as this setting does not deactivate
all encodings; More settings dedicated to each type of encoding
will likely be implemented in the future, as we make progress.

gdb/ChangeLog:

        * ada-lang.c (maint_set_ada_cmdlist, maint_show_ada_cmdlist):
        New static globals.
        (maint_set_ada_cmd, maint_show_ada_cmd): New functions.
        (ada_ignore_descriptive_types_p): New static global.
        (find_parallel_type_by_descriptive_type): Return immediately
        if ada_ignore_descriptive_types_p is set.
        (_initialize_ada_language): Register new commands "maintenance
        set ada", "maintenance show ada", "maintenance set ada
        ignore-descriptive-types" and "maintenance show ada
        ignore-descriptive-types".
        * NEWS: Add entry for new "maint ada set/show
        ignore-descriptive-types" commands.

gdb/doc/ChangeLog:

        * gdb.texinfo (Ada Glitches): Document the new "maint ada set/show
        ignore-descriptive-types". commands.