Avoid language-based lookups in startup path

Avoid language-based lookups in startup path

The previous patches are nearly enough to enable background DWARF
reading.  However, this hack in language_defn::get_symbol_name_matcher
causes an early computation of current_language:

  /* If currently in Ada mode, and the lookup name is wrapped in
     '<...>', hijack all symbol name comparisons using the Ada
     matcher, which handles the verbatim matching.  */
  if (current_language->la_language == language_ada
      && lookup_name.ada ().verbatim_p ())
    return current_language->get_symbol_name_matcher_inner (lookup_name);

I considered various options here -- reversing the order of the
checks, or promoting the verbatim mode to not be a purely Ada feature
-- but in the end found that the few calls to this during startup
could be handled more directly.

In the JIT code, and in create_exception_master_breakpoint_hook, gdb
is really looking for a certain kind of symbol (text or data) using a
linkage name.  Changing the lookup here is clearer and probably more
efficient as well.

In create_std_terminate_master_breakpoint, the lookup can't really be
done by linkage name (it would require relying on a certain mangling
scheme, and also may trip over versioned symbols) -- but we know that
this spot is C++-specific, and so the language ought to be temporarily
set to C++ here.

After this patch, the "file" case is much faster:

    (gdb) file /tmp/gdb
    2023-10-23 13:16:54.456 - command started
    Reading symbols from /tmp/gdb...
    2023-10-23 13:16:54.520 - command finished
    Command execution time: 0.225906 (cpu), 0.064313 (wall)