[lit] Keep original cfg file case around.

There's been some back and forth if the cfg paths in the
config_map should be normcase()d. The argument for is that
it allows using all-lower spelling in cmd on Windows, the
argument against that doing so is lossy.

Before the relative-paths-in-generated-lit.site.cfg.py work,
there was no downside to calling normcase(), but with it
we need a hack to recover the original case.

This time, normcase() the hashtable key, but store the original
cased key in addition to the value. This fixes both cons, at the
cost of a few bytes more memory.

Differential Revision: https://reviews.llvm.org/D78169