Change symbol_set_names to take an objfile_per_bfd_storage

This changes symbol_set_names to take an objfile_per_bfd_storage
argument, and updates the users.  It also changes PSYMBOL_SET_NAMES to
take this argument directly; I feel this clarifies the storage
location of objects created in psymtab.c.

gdb/ChangeLog
2019-01-10  Tom Tromey  <tom@tromey.com>

	* symtab.h (SYMBOL_SET_NAMES): Update.
	(symbol_set_names): Update.
	(MSYMBOL_SET_NAMES): Update.
	* symtab.c (symbol_set_names): Change argument to be an
	objfile_per_bfd_storage.
	* psymtab.c (add_psymbol_to_bcache): Update.
	* psympriv.h (PSYMBOL_SET_NAMES): Take per_bfd argument.