fix(cxx_indexer): normalize windows paths (#3742)

* fix(cxx_indexer): normalize windows paths

The implementation of Clang's FileManager::FixupRelativePath
assumes that all paths are in the host-machine style, so it's
currently hopeless to try and thread path style around the indexer.
Instead, since the only other path style right now is Windows,
turn Windows-style paths (though probably not UNC ones) into
POSIX-style paths (/c:/foo).

This doesn't mangle the various include arguments yet, so if
a compilation specifies an absolute path to an include directory
it won't have a good time.

CUs with missing or bad working directories are now only mildly
scolded instead of being rejected outright.
10 files changed