[Frontend] SetUpDiagnosticLog should handle unowned diagnostic consumer

in the compiler

The function SetUpDiagnosticLog that was called from createDiagnostics didn't
handle the case where the diagnostics engine didn't own the diagnostics consumer.
This is a potential problem for a clang tool, in particular some of the follow-up
patches for clang-scan-deps will need this fix.

Differential Revision: https://reviews.llvm.org/D63101

llvm-svn: 363009