Add psymtab_storage::allocate_dependencies

This adds a new method to psymtab_storage to allocate storage for
psymtab dependencies, then changes the symbol readers to use it.  This
has the effect of moving the storage to the psymtab storage obstack.

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

	* xcoffread.c (xcoff_end_psymtab): Use allocate_dependencies.
	* psymtab.h (psymtab_storage::allocate_dependencies): New method.
	* mdebugread.c (parse_partial_symbols): Use
	allocate_dependencies.
	* dwarf2read.c (dwarf2_create_include_psymtab): Use
	allocate_dependencies.
	(process_psymtab_comp_unit_reader)
	(build_type_psymtab_dependencies): Likewise.
	* dbxread.c (dbx_end_psymtab): Use allocate_dependencies.