Previously we set up the diagnostic options too early for these flags
to be honored, in order to diagnose problems in parsing the arguments
being passed to Clang. The solution is to decide that that diagnostic
engine is a temporary one and to create a proper one later. (And if
this sounds like a lousy solution, I don't disagree, but it's what
Clang does too.)
rdar://problem/39197895 and possibly something older but I can't find it
