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/Basic/VersionTuple.cpp b/lib/Basic/VersionTuple.cpp
index 4f479d0..8b781ab 100644
--- a/lib/Basic/VersionTuple.cpp
+++ b/lib/Basic/VersionTuple.cpp
@@ -28,9 +28,9 @@
 raw_ostream& clang::operator<<(raw_ostream &Out, 
                                      const VersionTuple &V) {
   Out << V.getMajor();
-  if (llvm::Optional<unsigned> Minor = V.getMinor())
+  if (Optional<unsigned> Minor = V.getMinor())
     Out << '.' << *Minor;
-  if (llvm::Optional<unsigned> Subminor = V.getSubminor())
+  if (Optional<unsigned> Subminor = V.getSubminor())
     Out << '.' << *Subminor;
   return Out;
 }