DiagnosticsEngine should clear DelayedDiagID before reporting the

delayed diagnostic

This fix avoids an infinite recursion that was uncovered in one of our internal
tests by r301992. The testcase is the most reduced version of that
auto-generated test.

This is an improved version of the reverted commit r302037. The previous fix
actually managed to expose another subtle bug whereby `fatal_too_many_errors`
error was reported twice, with the second report setting the
`FatalErrorOccurred` flag. That prevented the notes that followed the diagnostic
the caused `fatal_too_many_errors` to be emitted. This commit ensures that notes
that follow `fatal_too_many_errors` but that belong to the diagnostic that
caused `fatal_too_many_errors` won't be emitted by setting the
`FatalErrorOccurred` when emitting `fatal_too_many_errors`.

rdar://31962618

llvm-svn: 302151