[Driver] Change CLANG_ENABLE_OPAQUE_POINTERS_INTERNAL to affect driver default...

[Driver] Change CLANG_ENABLE_OPAQUE_POINTERS_INTERNAL to affect driver default instead of cc1 default

The current cc1 CLANG_ENABLE_OPAQUE_POINTERS=on default difference is not ideal
in that people contribute %clang_cc1 tests may assume the default ON behavior,
which will cause failures on systems set to OFF.

cc1 option default dependent on CMake options should be used prudently
(generally avoided). We prefer to limit target differences to Driver.

Change the CLANG_ENABLE_OPAQUE_POINTERS_INTERNAL mechanism introduced in D123122
to use a driver default instead. This is similar to the mechanism used for the
-flegacy-pass-manager transition to new PM transition.

Reviewed By: #opaque-pointers, rsmith, aeubanks

Differential Revision: https://reviews.llvm.org/D123744