[C++20][Modules][Driver][HU 4/N] Add fdirectives-only mode for preprocessing output.

When the -fdirectives-only option is used together with -E, the preprocessor
output reflects evaluation of if/then/else directives.

Thus it preserves macros that are still live after such processing.
This output can be consumed by a second compilation to produce a header unit.

We automatically invoke this (with -E) when we know that the job produces a
header unit so that the preprocessed output reflects the macros that will be
defined when the binary HU is emitted.

Differential Revision: https://reviews.llvm.org/D121591