[Clang] Enable opaque pointers by default

Enable opaque pointers by default in clang, which can be disabled
either via cc1 option -no-opaque-pointers or cmake flag
-DCLANG_ENABLE_OPAQUE_POINTERS=OFF.

See https://llvm.org/docs/OpaquePointers.html for context.

Differential Revision: https://reviews.llvm.org/D123300