[libc++][modules] Guard missing header validation on Windows. (#80478)

On Windows the libc++ test suite sees the MSVC STL headers and may
conclude these are libc++ headers when inspecting the name. Modules
guard against forgetting to export new headers. Finding MSVC STL's
headers gives false positives. Since the CI tests non-Windows platforms
too, the validation will be disabled on Windows.

Fixes: https://github.com/llvm/llvm-project/issues/79010



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Co-authored-by: Louis Dionne <ldionne.2@gmail.com>