[clang] Remove CLANG_ROUND_TRIP_CC1_ARGS and always roundtrip in +assert builds

This removes the ability to disable roundtripping in assert builds.
(Roundtripping happens by default in assert builds both before and after
this patch.)

The CLANG_ROUND_TRIP_CC1_ARGS was added as an escape hatch 9 months ago
in https://reviews.llvm.org/D97462, with a FIXME to remove it eventually.
It's probably time to remove it.

Differential Revision: https://reviews.llvm.org/D114120