Add hooks for typo correction to ExternalSemaSource, courtesy of Luke Zarko.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188196 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Sema/SemaLookup.cpp b/lib/Sema/SemaLookup.cpp
index e95ffce..d0bb9f6 100644
--- a/lib/Sema/SemaLookup.cpp
+++ b/lib/Sema/SemaLookup.cpp
@@ -3872,6 +3872,14 @@
                                  DeclContext *MemberContext,
                                  bool EnteringContext,
                                  const ObjCObjectPointerType *OPT) {
+  // Always let the ExternalSource have the first chance at correction, even
+  // if we would otherwise have given up.
+  if (ExternalSource) {
+    if (TypoCorrection Correction = ExternalSource->CorrectTypo(
+        TypoName, LookupKind, S, SS, CCC, MemberContext, EnteringContext, OPT))
+      return Correction;
+  }
+
   if (Diags.hasFatalErrorOccurred() || !getLangOpts().SpellChecking)
     return TypoCorrection();