Driver: Include driver diagnostics when we --serialize-diagnostics

Currently, when --serialize-diagnostics is passed this only includes
the diagnostics from clang -cc1, and driver diagnostics are
dropped. This causes issues for tools that use the serialized
diagnostics, since stderr is lost and these diagnostics aren't seen at
all.

We handle this by merging the diagnostics from the CC1 process and the
driver diagnostics into a single file when the driver invokes CC1.

Fixes rdar://problem/10585062