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/Analysis/BodyFarm.cpp b/lib/Analysis/BodyFarm.cpp
index 2779960..dda26bf 100644
--- a/lib/Analysis/BodyFarm.cpp
+++ b/lib/Analysis/BodyFarm.cpp
@@ -344,7 +344,7 @@
 Stmt *BodyFarm::getBody(const FunctionDecl *D) {
   D = D->getCanonicalDecl();
   
-  llvm::Optional<Stmt *> &Val = Bodies[D];
+  Optional<Stmt *> &Val = Bodies[D];
   if (Val.hasValue())
     return Val.getValue();