Ada/DWARF: Assume the Ada compiler produces descriptive type attributes

Ada/DWARF: Assume the Ada compiler produces descriptive type attributes

GCC was enhanced in 2011 to generate this attribute, so I think we can
now assume that it is available when using that compiler. Doing so
allows us to speed up what we call "parallel type" lookups when
processing certain types encoded using the GNAT encoding.

This patch changes need_gnat_info to always expect those attributes
to be generated when the language is Ada. This is an assumption
that on the surfcace looks like it might be a bit on the edge; but
in practice, it should be OK because this is only useful in the
context of handling GNAT-specific encodings. Other Ada compilers
would presumably produce debugging information using pure DWARF
constructs, so would not be impacted by this.

gdb/ChangeLog:

        * dwarf2read.c (need_gnat_info): Return nonzero if the cu's
        language is Ada.

Tested on x86_64-linux.