[ExternalASTMerger] Removed a move constructor to address MSVC build failure

llvm-svn: 299983
diff --git a/clang/lib/AST/ExternalASTMerger.cpp b/clang/lib/AST/ExternalASTMerger.cpp
index e47ed34..1f1a9b1 100644
--- a/clang/lib/AST/ExternalASTMerger.cpp
+++ b/clang/lib/AST/ExternalASTMerger.cpp
@@ -23,7 +23,7 @@
 
 template <typename T> struct Source {
   T t;
-  Source(T &&t) : t(std::move(t)) {}
+  Source(T t) : t(t) {}
   operator T() { return t; }
   template <typename U = T> U &get() { return t; }
   template <typename U = T> const U &get() const { return t; }