[ClangConfig] Unset some variables after we are done using them to configure ClangConfig.cmake.in files.

This ensures that the values do not bleed over in between computations. It may
make sense in the future to just refactor this code into functions to provide
"true scoping".

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@274179 91177308-0d34-0410-b5e6-96231b3b80d8