[clang] Add fp128 ABI tests for MinGW (NFC)

Add a test based on `win64-i128.c` with the current behavior of
`__float128`.

Author: tgross35

clang/test/CodeGen/win-fp128.c

@@ -0,0 +1,19 @@
+// RUN: %clang_cc1 -triple x86_64-windows-gnu -emit-llvm -o - %s \
+// RUN:    | FileCheck %s --check-prefix=CHECK-GNU64
+// __float128 is unsupported on MSVC
+
+__float128 fp128_ret(void) { return 0; }
+// CHECK-GNU64: define dso_local fp128 @fp128_ret()
+
+__float128 fp128_args(__float128 a, __float128 b) { return a * b; }
+// CHECK-GNU64: define dso_local fp128 @fp128_args(fp128 noundef %a, fp128 noundef %b)
+
+void fp128_vararg(int a, ...) {
+  // CHECK-GNU64-LABEL: define dso_local void @fp128_vararg
+  __builtin_va_list ap;
+  __builtin_va_start(ap, a);
+  __float128 i = __builtin_va_arg(ap, __float128);
+  // CHECK-GNU64: load ptr, ptr
+  // CHECK-GNU64: load fp128, ptr
+  __builtin_va_end(ap);
+}