[driver] Ignore -fno-var-tracking; it's a GCC option we don't support.

-fvar-tracking (which IIUC is on by default in GCC) will still generate
an error.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191439 91177308-0d34-0410-b5e6-96231b3b80d8