Make cp_remove_params return a gdb::unique_xmalloc_ptr

Make cp_remove_params return a gdb::unique_xmalloc_ptr

Use the type system instead of callers needing to know how the
returned string's memory is supposed to be managed.

gdb/ChangeLog:
2017-10-09  Pedro Alves  <palves@redhat.com>

	* cp-support.c (cp_remove_params): Return a gdb::unique_xmalloc_ptr.
	Use bool.
	(overload_list_add_symbol): Adjust to use gdb::unique_xmalloc_ptr.
	* cp-support.h (cp_remove_params): Now returns a
	gdb::unique_xmalloc_ptr.
	* dwarf2read.c (find_slot_in_mapped_hash): Now returns bool.
	Adjust to cp_remove_params returning a gdb::unique_xmalloc_ptr.
	* psymtab.c (psymtab_search_name): Adjust to cp_remove_params
	returning a gdb::unique_xmalloc_ptr.
	(lookup_partial_symbol): Adjust to use gdb::unique_xmalloc_ptr.
	* stack.c (find_frame_funname): Adjust to cp_remove_params
	returning a gdb::unique_xmalloc_ptr.