[CMake] Put controversial always-recheck-revision-number behind an option.

CMake does not have the ability to perform actions before calculating
dependencies, so it can't know whether it needs to rebuild clangBasic
to update for a new revision number. CLANG_ALWAYS_CHECK_VC_REV (off by
default) will cause clangBasic to always be dirty by deleting the
generated SVNVersion.inc after use; otherwise, SVNVersion.inc will
always be updated, but only included in the final binary when clangBasic
is rebuilt.

It'd be great to find a better way to do this, but hopefully this is
still an improvement over the complete lack of version information before.

llvm-svn: 190613