swift234

[ClangImporter] Address the "try to clear your module cache to see if things start working again"
mystical workaround that hits our dogfooders.


Address this by adding '-fmodules-validate-system-headers' to the clang importer, which rebuilds the clang modules if system headers change.
By my unscientific testing I didn't see significant difference for compile time on the template Cocoa project beyond noise level, and it shouldn't
be significant on the project level because clang module importing happens on per-swift-target basis, not per-swift-file.

Swift SVN r16482