[clang] [clang][modules] Add 'exclude umbrella' module map directive (PR #223706)

[Tor Arne Vestbø via cfe-commits]

torarnv wrote:

@Bigcheese, what do you think of this approach? It would save manual `exclude` directives for the Qt module maps, where the private headers live in e.g. `include/QtCore/6.13.0/QtCore/private/` while the public ones live in `include/QtCore`.

https://github.com/llvm/llvm-project/pull/223706