[Implicit Modules] Add -cc1 option -fmodules-strict-context-hash which...

[Implicit Modules] Add -cc1 option -fmodules-strict-context-hash which includes search paths and diagnostics.

This is a recommit of r375322 and r375327 with a fix for the Windows test breakage.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@375466 91177308-0d34-0410-b5e6-96231b3b80d8