Revert "Add Clang dependency to the check for Clang C++ headers."

This temporarily reverts commit r311733, because of bot breakage.
http://lab.llvm.org:8080/green/job/clang-stage1-configure-RA_build/38139/consoleFull#-256426522e9a0fee5-ebcc-4238-a641-c5aa112c323e

llvm-svn: 311757