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

[Ben Langmuir via cfe-commits]

https://github.com/benlangmuir commented:

Can you clarify how this bug occurs in terms of what information about the header is stored that causes us to get the wrong module? It seems bad that passing `nullptr` to `LookupFile` could cause incorrect behaviour and makes me wonder if we need to always trigger module lookup or if there is another way to fix this that would make it safe to not do module lookup here.

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