gdb: get_frame_language now takes a frame parameter.

gdb: get_frame_language now takes a frame parameter.

As part of a drive to remove deprecated_safe_get_selected_frame, make
the get_frame_language function take a frame parameter.  Given the name
of the function this actually seems to make a lot of sense.

The task of fetching a suitable frame is then passed to the calling
functions.  For get_frame_language there are not many callers, these are
updated to get the selected frame in a suitable way.

gdb/ChangeLog:

	* language.c (show_language_command): Find selected frame before
	asking for the language of that frame.
	(set_language_command): Likewise.
	* language.h (get_frame_language): Add frame parameter.
	* stack.c (get_frame_language): Add frame parameter, assert
	parameter is not NULL, update comment and reindent.
	* top.c (check_frame_language_change): Pass the selected frame
	into get_frame_language.