common: Use strcmp to compare location file names

common: Use strcmp to compare location file names

The logic to figure out where a missing #include should be inserted
uses pointer equality to check filenames -- the routine even says so.
But cpplib makes no such guarantee.  It happens to be true for input
that it preprocesses[* see line zero below], but is not true for
source that has already been preprocessed -- all those '# ...' line
directives produce disctinct filename strings.  That renders using
-fdirectives-only as a prescanning stage (as I understand some people
do), broken.

This patch changes to string comparisons, and explicitly rejects any
line-zero location map that occurs at the beginning of a file.  The
very first map of a file has a different string to the remaining maps,
and we never tripped on that because of the pointer comparison.  The
second testcase deploys -save-temps to cause an intermediate
preprocessed output that is read back.

	gcc/c-family/
	* c-common.c (try_to_locate_new_include_insertion_point): Use
	strcmp, not pointer equality.
	gcc/testsuite/
	* g++.dg/lookup/missing-std-include-10.h: New.
	* g++.dg/lookup/missing-std-include-10.C: New.
	* g++.dg/lookup/missing-std-include-11.C: New.