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/AST/ASTContext.cpp b/lib/AST/ASTContext.cpp
index 952162e..a9df146 100644
--- a/lib/AST/ASTContext.cpp
+++ b/lib/AST/ASTContext.cpp
@@ -3234,7 +3234,7 @@
 }
 
 QualType ASTContext::getPackExpansionType(QualType Pattern,
-                                      llvm::Optional<unsigned> NumExpansions) {
+                                          Optional<unsigned> NumExpansions) {
   llvm::FoldingSetNodeID ID;
   PackExpansionType::Profile(ID, Pattern, NumExpansions);