treat Ada DW_TAG_unspecified_type DIEs as stub types

treat Ada DW_TAG_unspecified_type DIEs as stub types

Consider the gdb.ada/taft_type.exp testcase, which exercises
the situation where a variable is defined using a type which
is a pointer to an incomplete type, with the actual type
definition being provided by another unit. Up to now, the
strategy used by GNAT when generating the DWARF debugging info
was to produce a incomplete DW_TAG_enumeration_type DIE with
a DW_AT_declaration flag attached to it:

        .uleb128 0x4    # (DIE (0x3e) DW_TAG_enumeration_type)
        .long   .LASF4  # DW_AT_name: "pck__empty"
                        # DW_AT_declaration

However, a more standard way for the compiler to describe
this kind of type is to use the DW_TAG_unspecified_type tag.
When the compiler is enhanced to do so, we'll need to treat
such types as stubs -- we only do so with types from Ada units,
however, as the meaning of this TAG is intentionally left
permissive and language-specific by the DWARF standard.

Without this patch, running the testcase above with an enhanced
compiler now yields:

    (gdb) print w.e.all
    Attempt to dereference a generic pointer.
    FAIL: gdb.ada/taft_type.exp: print w.e.all

gdb/ChangeLog:

	* dwarf2read.c (read_unspecified_type): Treat
        DW_TAG_enumeration_type DIEs from Ada units as stubs.

Tested on x86_64-linux, fixes the FAIL in gdb.ada/taft_type.exp above.