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.
When the -fdirectives-only option is used together with -E, the preprocessor
output reflects evaluation of if/then/else directives.
As such, it preserves defines and undefs of macros that are still live after
such processing. The intent is that this output could be consumed as input
to generate considered a C++20 header unit.
We strip out any (unused) defines that come from built-in, built-in-file or
command line; these are re-added when the preprocessed source is consumed.
Differential Revision: https://reviews.llvm.org/D121099
