[clang][driver] Ensure we don't accumulate entries in -MJ files

Previously, each job would overwrite the -MJ file. This didn't quite work for Clang invocations with multiple architectures, which got fixed in D121997 by always appending to the -MJ file. That's not correct either, since the file would grow indefinitely on subsequent Clang invocations. This patch ensures the driver always removes the file before jobs fill it in by appending.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D128098