Don't run -Wunreachable-code on template instantiations.  Different instantiations may produce different unreachable code results, and it is very difficult for us to prove that ALL instantiations of a template have specific unreachable code.  If we come up with a better solution, then we can revisit this, but this approach will at least greatly reduce the noise of this warning for code that makes use of templates.

llvm-svn: 145520