[Clang] Limit -fintegrated-cc1 to only one TU

As discussed in https://reviews.llvm.org/D74447, this patch disables integrated-cc1 behavior if there's more than one job to be executed. This is meant to limit memory bloating, given that currently jobs don't clean up after execution (-disable-free is always active in cc1 mode).

I see this behavior as temporary until release 10.0 ships (to ease merging of this patch), then we'll reevaluate the situation, see if D74447 makes more sense on the long term.

Differential Revision: https://reviews.llvm.org/D74490