cppinit.c (cpp_read_main_file): Split into two functions: Distribute _cpp_stack_file...

cppinit.c (cpp_read_main_file): Split into two functions: Distribute _cpp_stack_file call over the two functions.

	* cppinit.c (cpp_read_main_file):  Split into two functions:
	Distribute _cpp_stack_file call over the two functions.
	(cpp_find_main_file):  New function.
	Don't call _cpp_do_file_change even if working_directory flag set.
	(cpp_push_main_file):  New function.
	* cppfiles.c (_cpp_find_failed):  New helper function.
	(find_file):  Made non-static and renamed to _cpp_find_file.
	(_cpp_stack_file):  No longer needed.  But note the following.
	(stack_file):  Made non-static and renamed to _cpp_stack_file.
	* fix-header.c (cpp_read_main_file):  Replace cpp_read_main_file
	call with calls to cpp_find_main_file and cpp_push_main_file.
	(search_path_head):  If there is no current buffer, use main_file.
	* cpphash.h:  Update function declarations.
	* cpplib.h:  Update function declarations.

From-SVN: r72014