Include llvm::Optional in clang/Basic/LLVM.h

Post-commit CR feedback from Jordan Rose regarding r175594.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175679 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Sema/Sema.cpp b/lib/Sema/Sema.cpp
index a52d1f2..8c79d82 100644
--- a/lib/Sema/Sema.cpp
+++ b/lib/Sema/Sema.cpp
@@ -838,7 +838,7 @@
   // eliminnated. If it truly cannot be (for example, there is some reentrancy
   // issue I am not seeing yet), then there should at least be a clarifying
   // comment somewhere.
-  if (llvm::Optional<TemplateDeductionInfo*> Info = isSFINAEContext()) {
+  if (Optional<TemplateDeductionInfo*> Info = isSFINAEContext()) {
     switch (DiagnosticIDs::getDiagnosticSFINAEResponse(
               Diags.getCurrentDiagID())) {
     case DiagnosticIDs::SFINAE_Report: