Fix "start" for D, Rust, etc

Fix "start" for D, Rust, etc

The new DWARF indexer broke "start" for some languages.

For D, it is broken because, while the code in cooked_index_shard::add
specifically excludes Ada, it fails to exclude D.  This means that the
C "main" will be detected as "main" here -- whereas what is intended
is for the code in find_main_name to use d_main_name to find the name.

The Rust compiler, on the other hand, uses DW_AT_main_subprogram.
However, the code in dwarf2_build_psymtabs_hard fails to create a
fully-qualified name, so the name always ends up as plain "main".

For D and Ada, a very simple approach suffices: remove the check
against "main" from cooked_index_shard::add.  This also has the
benefit of slightly speeding up DWARF indexing.  I assume this
approach will work for Pascal and Modula-2 as well, but I don't have a
way to test those at present.

For Rust, though, this is not sufficient.  And, computing the
fully-qualified name in dwarf2_build_psymtabs_hard will crash, because
cooked_index_entry::full_name uses the canonical name -- and that is
not computed until after canonicalization.

However, we don't want to wait for canonicalization to be done before
computing the main name.  That would remove any benefit from doing
canonicalization is the background.

This patch solves this dilemma by noticing that languages using
DW_AT_main_subprogram are, currently, disjoint from languages
requiring canonicalization.  Because of this, we can add a parameter
to full_name to let us avoid crashes, slowdowns, and races here.

This is kind of tricky and ugly, so I've tried to comment it
sufficiently.

While doing this, I had to change gdb.dwarf2/main-subprogram.exp.  A
different possibility here would be to ignore the canonicalization
needs of C in this situation, because those only affect certain types.
However, I chose this approach because the test case is artificial
anyhow.

A long time ago, in an earlier threading attempt, I changed the global
current_language to be a function (hidden behind a macro) to let us
attempt lazily computing the current language.  Perhaps this approach
could still be made to work.  However, that also seemed rather tricky,
more so than this patch.

Reviewed-By: Andrew Burgess <aburgess@redhat.com>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30116
(cherry picked from commit 47fe57c92810c7302bb80eafdec6f4345bcc69c8)