[clang] [clang][deps] Fix `__has_include` behavior with umbrella headers (PR #70144)

[Ben Langmuir via cfe-commits]

https://github.com/benlangmuir approved this pull request.

> Yeah. I did try to fix up all calls to LookupFile to perform module map lookup, but a bunch of tests started failing (mostly standard C++ modules tests IIRC), so there's probably more nuance required there.

Okay, I do think this is worth fixing long term, but I don't want to block on it.  Your change LGTM in the meantime.

https://github.com/llvm/llvm-project/pull/70144