CMake: avoid a reconfigure loop from r213091

Removing the native CMakeCache.txt causes the target to get re-run needlessly
on some systems. We'll want another solution for that part of the fix.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213099 91177308-0d34-0410-b5e6-96231b3b80d8