Simplify minsym iteration

Simplify minsym iteration

This simplifies the minimal symbol iterator, by using
minimal_symbol_count and just doing a somewhat ordinary array-like
iteration.  array_view is nearly usable, except that it is more
convenient for this iterator to return pointers rather than
references.

gdb/ChangeLog
2019-01-16  Tom Tromey  <tom@tromey.com>

	* objfiles.h (class objfile_msymbols) <iterator>: Change argument
	type.  Remove no-argument constructor.
	<iterator::operator++>: Simplify.
	<begin>: Update.
	<end>: Use minimal_symbol_count.