We support a mode where the Clang regression tests are run but not the

We support a mode where the Clang regression tests are run but not the
Clang unit tests. It's not clear why we support this mode in builds
where LLVM is available (LLVM itself does not), but at least this makes
us support it correctly.

This also fixes a long-standing bug where we would pass the unit test
param flag to lit in the standalone build even though the standalone
build *never* has the unittests built and ready for testing.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@159594 91177308-0d34-0410-b5e6-96231b3b80d8