Use outermost_context_p in more places

Use outermost_context_p in more places

This changes a few explicit checks of context_stack_depth to use
outermost_context_p instead.  This simplifies some future work.

gdb/ChangeLog
2018-07-16  Tom Tromey  <tom@tromey.com>

	* xcoffread.c (read_xcoff_symtab): Use outermost_context_p.
	* dwarf2read.c (using_directives, new_symbol): Use
	outermost_context_p.
	* dbxread.c (process_one_symbol): Use outermost_context_p.
	* coffread.c (coff_symtab_read): Use outermost_context_p.