Fix forwarding of -fpack-struct from driver to CC1, and add a test.

Fix forwarding of -fpack-struct from driver to CC1, and add a test.

-fpack-struct's handling has changed in CC1 (one of only two flags that needed changing) because the driver treats "-fpack-struct" as a boolean flag, and CC1 (did) treat it as an option with a separated value.

This change causes -fpack-struct=X to be forwarded correctly to -fpack-struct=X instead of erroneously to "-fpack-struct X"

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@155981 91177308-0d34-0410-b5e6-96231b3b80d8