Rather than duplicating extension diagnostics to allow them to cause a

Rather than duplicating extension diagnostics to allow them to cause a
substitution failure, allow a flag to be set on the Diagnostic object,
to mark it as 'causes substitution failure'.

Refactor Diagnostic.td and the tablegen to use an enum for SFINAE behavior
rather than a bunch of flags.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194444 91177308-0d34-0410-b5e6-96231b3b80d8