[clang][driver] Introduce new -fdriver-only flag

This patch introduces the new -fdriver-only flag which instructs Clang to only execute the driver logic without running individual jobs. In a way, this is very similar to -###, with the following differences:
 * it doesn't automatically print all jobs,
 * it doesn't avoid side effects (e.g. it will generate compilation database when -MJ is specified).

This flag will be useful in testing D121997.

Reviewed By: dexonsmith, egorzhdan

Differential Revision: https://reviews.llvm.org/D127408