Revert "Ensure .inc files are regenerated when llvm/clang tblgen binaries change"

This reverts commit ab80f0b21fbb9c44d40e6f7a99090188f4ed2f71. The intent
of this change was to avoid possible compilation errors when certain
.inc files were not regenerated, but the method turns out to cause way
more rebuilds than anticipated. Another method will have to be found,
and in the mean time, WITH_CLEAN is the solution that always works.

Fixes: ab80f0b21fbb