Use bitset for demangled_hash_languages

Use bitset for demangled_hash_languages

I noticed that objfile_per_bfd_storage::demangled_hash_languages is a
std::vector, which seemed quite large for something that,
fundamentally, can be represented as a bitset.  This patch
reimplements it as a std::bitset.

gdb/ChangeLog
2019-03-15  Tom Tromey  <tom@tromey.com>

	* objfiles.h (struct objfile_per_bfd_storage)
	<demangled_hash_languages>: Now a bitset.
	* minsyms.c (add_minsym_to_demangled_hash_table): Update.
	(lookup_minimal_symbol): Update.