libcpp: Directly peek for initial line marker

Using the tokenizer to sniff for an initial line marker for
preprocessed input is a little brittle, particularly with
-fdirectives-only.  If there is no marker we'll happily munch initial
comments.  This patch directly sniffs the buffer.  This is safe
because the initial line marker was machine generated and must be
right at the beginning of the file.  Anything else is not such a line
marker.  The same is true for the initial directory marker.  For that
tokenizing the string is simplest, but at that point it's either a
regular line marker or a directory marker.  If it's a regular marker,
unwinding tokens is fine.

	libcpp/
	* internal.h (enum include_type): Rename IT_MAIN_INJECT to
	IT_PRE_MAIN.
	* init.c (cpp_read_main_file): If there is no line marker, adjust
	the initial line marker.
	(read_original_filename): Return bool, peek the buffer directly
	before trying to tokenize.
	(read_original_directory): Likewise.  Directly prod the string
	literal.
	* files.c (_cpp_stack_file): Adjust for IT_PRE_MAIN change.