Intrinsics shouldn't see struct padding, so use a special TargetABI for them

that removes it.
This unbreaks the `llvm_*_with_overflow` intrinsics.

tests/mini/intrinsics_ovf.d

@@ -0,0 +1,77 @@
+module intrinsics_ovf;
+
+
+//version = PRINTF;
+
+version(PRINTF)
+    extern(C) int printf(char*, ...);
+
+
+import ldc.intrinsics;
+
+
+int saddo(int a, int b, out bool overflow) {
+    auto Result = llvm_sadd_with_overflow(a, b);
+    overflow = Result.overflow;
+    return Result.result;
+}
+
+int uaddo(int a, int b, out bool overflow) {
+    auto Result = llvm_uadd_with_overflow(a, b);
+    overflow = Result.overflow;
+    return Result.result;
+}
+
+int smulo(int a, int b, out bool overflow) {
+    auto Result = llvm_smul_with_overflow(a, b);
+    overflow = Result.overflow;
+    return Result.result;
+}
+
+/*
+uint umulo(uint a, uint b, out bool overflow) {
+    auto Result = llvm_umul_with_overflow(a, b);
+    overflow = Result.overflow;
+    return Result.result;
+}
+*/
+
+void test(int function(int, int, out bool) fn,
+          int a, int b, int result_e, bool ovf_e) {
+    version(PRINTF)
+        printf("%8x :: %8x :: %8x :: %.*s\n", a, b, result_e,
+                (ovf_e ? "true" : "false"));
+    
+    bool ovf;
+    int result = fn(a, b, ovf);
+    
+    version(PRINTF)
+        printf("____________________    %8x :: %.*s\n", result,
+                (ovf ? "true" : "false"));
+    
+    assert(ovf == ovf_e);
+    assert(result == result_e);
+}
+
+void main() {
+    test(&saddo, int.min, int.min, int.min + int.min, true);
+    test(&saddo, int.min, int.max, int.min + int.max, false);
+    test(&saddo, 1, int.max, 1 + int.max, true);
+    test(&saddo, 1, 2, 3, false);
+    test(&saddo, -1, -2, -3, false);
+    
+    test(&uaddo, 0, uint.max, 0 + uint.max, false);
+    test(&uaddo, 1, uint.max, 1 + uint.max, true);
+    test(&uaddo, 1, 2, 3, false);
+
+    test(&smulo, int.min, int.min, int.min * int.min, true);
+    test(&smulo, int.min, int.max, int.min * int.max, true);
+    test(&smulo, int.max, int.max, int.max * int.max, true);
+    test(&smulo, 1, int.max, 1 * int.max, false);
+    test(&smulo, 2, int.max/2, 2 * (int.max/2), false);
+    test(&smulo, 2, int.max/2 + 1, 2 * (int.max/2 + 1), true);
+    test(&smulo, 2, int.min/2, 2 * (int.min/2), false);
+    test(&smulo, 2, int.min/2 - 1, 2 * (int.min/2 - 1), true);
+    test(&smulo, 1, 2, 2, false);
+    test(&smulo, -1, -2, 2, false);
+}