[clang] [clang][Interp] Implement IntegralAP subtraction (PR #71648)

[Aaron Ballman via cfe-commits]

================
@@ -256,28 +255,23 @@ template <bool Signed> class IntegralAP final {
   }
 
 private:
-  static bool CheckAddUB(const IntegralAP &A, const IntegralAP &B,
-                         unsigned BitWidth, IntegralAP *R) {
-    if (!A.isSigned()) {
-      R->V = A.V + B.V;
+  template <template <typename T> class Op>
+  static bool CheckAddSubUB(const IntegralAP &A, const IntegralAP &B,
+                            unsigned BitWidth, IntegralAP *R) {
+    if constexpr (!Signed) {
+      auto UOp = Op<APInt>();
+      R->V = UOp(A.V, B.V);
       return false;
     }
 
-    const APSInt &LHS = APSInt(A.V, A.isSigned());
-    const APSInt &RHS = APSInt(B.V, B.isSigned());
-
-    APSInt Value(LHS.extend(BitWidth) + RHS.extend(BitWidth), false);
+    auto SOp = Op<APSInt>();
+    const APSInt &LHS = A.toAPSInt();
+    const APSInt &RHS = B.toAPSInt();
+    APSInt Value(SOp(LHS.extend(BitWidth), RHS.extend(BitWidth)), false);
----------------
AaronBallman wrote:

```suggestion
    const APSInt &LHS = A.toAPSInt();
    const APSInt &RHS = B.toAPSInt();
    APSInt Value(Op<APSInt>{}(LHS.extend(BitWidth), RHS.extend(BitWidth)), 
false);
```

https://github.com/llvm/llvm-project/pull/71648
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