[Clang] Add VerboseOutputStream to CompilerInstance

Remove one instance of a hardcoded output stream in
CompilerInstance::ExecuteAction. There are still other cases of output
being hard-coded to standard streams in ExecuteCompilerInvocation, but
this patch covers the case when no flags like -version or -help are
passed, namely the "X warnings and Y errors generated." diagnostic.

Differential Revision: https://reviews.llvm.org/D53768

llvm-svn: 375442