Migrate varargs.cpp from llvm/test/FrontendC++.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@137764 91177308-0d34-0410-b5e6-96231b3b80d8

test/CodeGenCXX/varargs.cpp

+// RUN: %clang_cc1 -emit-llvm %s -o - | FileCheck %s
+// rdar://7309675
+// PR4678
+
+// test1 should be compmiled to be a varargs function in the IR even
+// though there is no way to do a va_begin.  Otherwise, the optimizer
+// will warn about 'dropped arguments' at the call site.
+
+// CHECK: define i32 @_Z5test1z(...)
+int test1(...) {
+  return -1;
+}
+
+// CHECK: call i32 (...)* @_Z5test1z(i32 0)
+void test() {
+  test1(0);
+}