[OpaquePtr][Clang] Add CLANG_ENABLE_OPAQUE_POINTERS cmake option

This option controls whether -opaque-pointers or -no-opaque-pointers
is the default. Once opaque pointers are enabled by default, this
will provide a simple way to temporarily opt-out of the change.

Differential Revision: https://reviews.llvm.org/D123122

GitOrigin-RevId: 5390606aa963a7b415da65aa8120efbbafd30401
3 files changed