Driver: Record that we're in crashdump and push flags to ConstructJob

It's more flexible and arguably better layering to set flags to modify
compiling for diagnostics in the CC1 job themselves, rather than
tweaking the driver flags and letting them propagate.

There is one visible change this causes: crash report files will now
get preprocessed names (.i and friends).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211411 91177308-0d34-0410-b5e6-96231b3b80d8