Change minimal_symbol_reader to store objfile

Change minimal_symbol_reader to store objfile

This changes minimal_symbol_reader to require the objfile to be
passed to the constructor.  The class now records the objfile and
automatically uses it later in "install".

This is a minor cleanup that will come in useful in the next patch.
It is separate from the first patch to keep that one a bit simpler to
understand.

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

	* xcoffread.c (xcoff_initial_scan): Update.
	* mipsread.c (mipscoff_symfile_read): Update.
	* minsyms.c (minimal_symbol_reader): Add obj argument.
	Initialize member.
	(install): Remove objfile argument.  Update.
	* mdebugread.c (elfmdebug_build_psymtabs): Update.
	* machoread.c (macho_symfile_read): Update.
	* elfread.c (elf_read_minimal_symbols): Update.
	* dbxread.c (dbx_symfile_read): Update.
	* coffread.c (coff_symfile_read): Update.
	* minsyms.h (minimal_symbol_reader): Add m_objfile member.
	(constructor): Add objfile argument.
	(minimal_symbol_reader::install): Remove objfile argument.