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

Post-commit CR feedback from Jordan Rose regarding r175594.

llvm-svn: 175679
diff --git a/clang/lib/StaticAnalyzer/Checkers/NSErrorChecker.cpp b/clang/lib/StaticAnalyzer/Checkers/NSErrorChecker.cpp
index 9e2202b..9f01522 100644
--- a/clang/lib/StaticAnalyzer/Checkers/NSErrorChecker.cpp
+++ b/clang/lib/StaticAnalyzer/Checkers/NSErrorChecker.cpp
@@ -186,7 +186,7 @@
 static QualType parameterTypeFromSVal(SVal val, CheckerContext &C) {
   const StackFrameContext *
     SFC = C.getLocationContext()->getCurrentStackFrame();
-  if (llvm::Optional<loc::MemRegionVal> X = val.getAs<loc::MemRegionVal>()) {
+  if (Optional<loc::MemRegionVal> X = val.getAs<loc::MemRegionVal>()) {
     const MemRegion* R = X->getRegion();
     if (const VarRegion *VR = R->getAs<VarRegion>())
       if (const StackArgumentsSpaceRegion *