Introduce minimal_symbol_reader

Introduce minimal_symbol_reader

This patch introduced minimal_symbol_reader, a RAII-based class which
replaces the current make_cleanup_discard_minimal_symbols.

2016-10-21  Tom Tromey  <tom@tromey.com>

	* xcoffread.c (xcoff_initial_scan): Use
	minimal_symbol_reader.
	* mipsread.c (mipscoff_symfile_read): Use
	minimal_symbol_reader.
	* minsyms.h (minimal_symbol_reader): New class.
	(init_minimal_symbol_collection)
	(make_cleanup_discard_minimal_symbols, install_minimal_symbols):
	Don't declare.
	* minsyms.c (minimal_symbol_reader): Renamed from
	init_minimal_symbol_collection, turned into constructor.
	(~minimal_symbol_reader): Renamed from
	do_discard_minimal_symbols_cleanup, turned into destructor.
	(make_cleanup_discard_minimal_symbols): Remove.
	(minimal_symbol_reader::install): Rename form
	install_minimal_symbols.
	* mdebugread.c (elfmdebug_build_psymtabs): Use
	minimal_symbol_reader.
	* machoread.c (macho_symfile_read): Use
	minimal_symbol_reader.
	* elfread.c (elf_read_minimal_symbols): Use
	minimal_symbol_reader.
	* dbxread.c (dbx_symfile_read): Use minimal_symbol_reader.
	* coffread.c (coff_symfile_read): Use
	minimal_symbol_reader.