Prospective -fsanitize=memory build fix following r212586

This -f group flag appears to influence linker flags, breaking the usual rules
and causing CMake's link invocation to fail during feature detection due to
missing link dependencies (msan_*).

Let's forcibly add it for now to get things the way they were before feature
detection started working.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212590 91177308-0d34-0410-b5e6-96231b3b80d8