[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
(cherry picked from commit 20f1abe306d030e99f56185a3aa077ffadf59b8a)