Don't use the mutex for each symbol_set_names call

Don't use the mutex for each symbol_set_names call

This avoids the lock contention that was seen with tromey's patch (it
moves it to a once- per-thread lock call from a once-per-symbol call).

It speeds up "attach to Chrome's content_shell binary" from 50 sec -> 30
sec (32%).

See https://sourceware.org/ml/gdb-patches/2019-10/msg00087.html for some
more speed measurements.

gdb/ChangeLog:

2019-09-28  Christian Biesinger  <cbiesinger@google.com>

	* minsyms.c (minimal_symbol_reader::install): Only do
	demangling on the background thread; still call
	symbol_set_names on the main thread.
	* symtab.c (symbol_find_demangled_name): Make public,
	so that minsyms.c can call it.
	(symbol_set_names): Remove mutex. Avoid demangle call
	if the demangled name is already set.
	* symtab.h (symbol_find_demangled_name): Make public.