Relax recently added clang version checks.

You can override the value of these during CMake, and we often use sentinels
with more than one digit (not to mention our actual Clang being 700.whatever).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@260596 91177308-0d34-0410-b5e6-96231b3b80d8