Improve specificity in the diag-mapping2.c test

Make sure grep catches the correct diagnostic, otherwise this test fails to
detect broken functionality.
Implement -Wdate-time preprocessor warning

This GCC warning is useful for validating reproducible builds
and might help when tracking down issues with modules too.
Show -Wdate-time in system headers

Anyone enabling this warning would expect to hear about all occurrences
including those in system headers that can cause non-reproducible builds.

To achieve this, rework ShowInSystemHeader to remove broken unused mapping code
that didn't make sense with a simpler and correct scheme.