There is a long-standing FIXME in `HeaderSearch.cpp` to use the path separator preferred by the platform instead of forward slash. There was an attempt to fix that (1cf6c28a) which got reverted (cf385dc8). I couldn't find an explanation, but my guess is that some tests assuming forward slash started failing.
This commit fixes tests with that assumption.
This is intended to be NFC, but there are two exceptions to that:
* Some diagnostic messages might now contain backslash instead of forward slash.
* Arguments to the "-remap-file" option that use forward slash might stop kicking in. Separators between potential includer path and header name need to be replaced by backslash in that case.
`HeaderSearch` currently assumes `LookupFileCache` is eventually populated in `LookupFile`. However, that's not always the case with `-fms-compatibility` and its early returns.
This patch adds a defensive check that the iterator pulled out of the cache is actually valid before using it.
(This bug was introduced in D119721. Before that, the cache was initialized to `0` - essentially the `search_dir_begin()` iterator.)
Reviewed By: dexonsmith, erichkeane
Differential Revision: https://reviews.llvm.org/D122237
