[OpaquePtrs][Clang] Add -opaque-pointers/-no-opaque-pointers cc1 options

This adds cc1 options for enabling and disabling opaque pointers
on the clang side. This is not super useful now (because
-mllvm -opaque-pointers and -Xclang -opaque-pointers have the same
visible effect) but will be important once opaque pointers are
enabled by default in clang. In that case, it will only be
possible to disable them using the cc1 -no-opaque-pointers option.

Differential Revision: https://reviews.llvm.org/D123034