[clang] [clang] Add support for relaxed constexpr evaluation to interpreter (PR #217311)

[via cfe-commits]

================
@@ -2713,9 +2715,13 @@ bool InvalidShuffleVectorIndex(InterpState &S, CodePtr OpPC, uint32_t Index) {
 bool CheckPointerToIntegralCast(InterpState &S, CodePtr OpPC,
                                 const Pointer &Ptr, unsigned BitWidth) {
   SourceInfo E = S.Current->getSource(OpPC);
-  S.CCEDiag(E, diag::note_constexpr_invalid_cast)
-      << 2 << S.getLangOpts().CPlusPlus << S.Current->getRange(OpPC);
 
+  S.CCEDiag(E, diag::note_constexpr_invalid_cast_ptrtoint)
+      << diag::ConstexprInvalidCastKind::ThisConversionOrReinterpret
+      << S.getLangOpts().CPlusPlus << S.Current->getRange(OpPC);
+  APValue V = Ptr.toAPValue(S.getASTContext());
+  if (V.getLValueBase())
+    S.CCEDiag(E, diag::note_constexpr_has_lvalue) << S.Current->getRange(OpPC);
----------------
eleviant wrote:

Here we check that the base of LValue is not null. In this sense the declaration below won't pass the check:
```
struct Foo object;
constexpr uintptr_t v = (uintptr_t)&object
```
and this one will:
```
constexpr uintptr_t offtset = (uintptr)&(((Foo*)nullptr)->field);
```
I don't think `Ptr.isZero()` would work, because in both cases pointer is not zero.

https://github.com/llvm/llvm-project/pull/217311