Introduce objfile::reset_psymtabs

This introduces a new method, objfile::reset_psymtabs, and changes
reread_symbols to use it.  This method simply destroys the existing
partial symbols and recreates the psymtab_storage object.

This patch fixes a latent bug -- namely, that reread_symbols should
clear objfile::psymbol_map, but does not.  I can submit that
separately if you'd prefer.

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

	* symfile.c (reread_symbols): Call objfile->reset_psymtabs.
	* objfiles.h (objfile::reset_psymtabs): New method.