[clang] [modulemap] Add -fno-implicit-module-map-file= to exclude from implicit discovery (PR #208191)

[Tor Arne Vestbø via cfe-commits]

torarnv wrote:

Thanks for all your help @cyndyishida! I went with the VFS overlay approach in https://codereview.qt-project.org/c/qt/qtbase/+/756576 for C++ consumers that explicitly use `-fcxx-modules`. 

I looked into shipping module maps for Swift with different names or non-standard locations, as an alternate solution, but picking up those module maps for Swift consumption was tricky, needing `unsafeFlags`, which doesn't propagate to consumers, or relying on module maps that pointed to headers outside of its directory, which is deprecated and may be a hard error at some point.

I'll let the Qt change run though CI and validate it end to end for Swift consumption with all of Qt, but if it works, I think we can close this PR :)

https://github.com/llvm/llvm-project/pull/208191