Avoid double-decoding in ada_add_global_exceptions

Avoid double-decoding in ada_add_global_exceptions

I noticed that ada_add_global_exceptions calls ada_decode on
'search_name' -- and then passes this to name_matches_regex, which
also calls ada_decode.

name_matches_regex is also used later, where the result of
'natural_name ()' is passed to it -- but natural_name also calls
ada_decode.

So, I think the call to ada_decode in name_matches_regex is redundant.
This patch removes it, and turns name_matches_regex into an inner
function to avoid propagating its use.

Note that, right now, the DWARF implementation of
expand_symtabs_matching does not in fact pass an encoded name to this
callback.  So, this code remains slightly (but currently harmlessly)
wrong.  expand_symtabs_matching is fixed by another pending series of
mine.