Use v.data() instead of &v[0] when SmallVector v might be empty.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@72210 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/AST/Builtins.cpp b/lib/AST/Builtins.cpp
index ca75c5d..8368feb 100644
--- a/lib/AST/Builtins.cpp
+++ b/lib/AST/Builtins.cpp
@@ -285,6 +285,6 @@
   // handle untyped/variadic arguments "T c99Style();" or "T cppStyle(...);".
   if (ArgTypes.size() == 0 && TypeStr[0] == '.')
     return Context.getFunctionNoProtoType(ResType);
-  return Context.getFunctionType(ResType, &ArgTypes[0], ArgTypes.size(),
+  return Context.getFunctionType(ResType, ArgTypes.data(), ArgTypes.size(),
                                  TypeStr[0] == '.', 0);
 }