Support non-owned DiagnosticConsumer in SetupSerializedDiagnostics

This fixes an assertion when running clang-tidy on a file having
--serialize-diagnostics in compiler options. Committing a regression test
for clang-tidy separately.

Patch by Aaron Wishnick!


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221884 91177308-0d34-0410-b5e6-96231b3b80d8