compile commands header to source heuristic lower-cases filenames before inferring file types

This leads to ".C" files being rewritten as ".c" files and being inferred to be "c" files as opposed to "c++" files.

Fixes https://github.com/clangd/clangd/issues/1108

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D124262