Convert symbol_set_demangled_name to a method

Convert symbol_set_demangled_name to a method

This changes symbol_set_demangled_name to be a method on
general_symbol_info, and updates the users.

gdb/ChangeLog
2020-04-24  Tom Tromey  <tom@tromey.com>

	* symtab.h (struct general_symbol_info) <set_demangled_name>: New
	method.
	(symbol_set_demangled_name): Don't declare.
	* symtab.c (general_symbol_info::set_demangled_name): Rename from
	symbol_set_demangled_name.
	(general_symbol_info::set_language)
	(general_symbol_info::compute_and_set_names): Update.
	* minsyms.c (minimal_symbol_reader::install): Update.
	* dwarf2/read.c (new_symbol): Update.