[clang] Handle constant "pointers" for `__atomic_always_lock_free`/`__atomic_is_lock_free`. (PR #99340)

[James Y Knight via cfe-commits]

================
@@ -124,6 +124,24 @@ _Static_assert(__atomic_always_lock_free(4, &i64), "");
 _Static_assert(!__atomic_always_lock_free(8, &i32), "");
 _Static_assert(__atomic_always_lock_free(8, &i64), "");
 
+// Validate use with fake pointers constants. This mechanism is used to allow
+// validating atomicity of a given size and alignment.
+_Static_assert(__atomic_is_lock_free(1, (void*)1), "");
+_Static_assert(__atomic_is_lock_free(1, (void*)-1), "");
+_Static_assert(__atomic_is_lock_free(4, (void*)2), ""); // expected-error {{not an integral constant expression}}
+_Static_assert(__atomic_is_lock_free(4, (void*)-2), ""); // expected-error {{not an integral constant expression}}
+_Static_assert(__atomic_is_lock_free(4, (void*)4), "");
+_Static_assert(__atomic_is_lock_free(4, (void*)-4), "");
+
+_Static_assert(__atomic_always_lock_free(1, (void*)1), "");
+_Static_assert(__atomic_always_lock_free(1, (void*)-1), "");
+_Static_assert(!__atomic_always_lock_free(4, (void*)2), "");
+_Static_assert(!__atomic_always_lock_free(4, (void*)-2), "");
+_Static_assert(__atomic_always_lock_free(4, (void*)4), "");
+_Static_assert(__atomic_always_lock_free(4, (void*)-4), "");
----------------
jyknight wrote:

Neat, found a pre-existing bug: `_Static_assert(__atomic_always_lock_free(2, "string"), "");` triggers an assertion failure in clang at head because the type of `"string"` is ConstantArrayType not PointerType, in this cast:
https://github.com/llvm/llvm-project/blob/83fbd79319a4d997520c85ab41997692a58cd958/clang/lib/AST/ExprConstant.cpp#L12960

The argument AST looks like:
```
(gdb) p E->getArg(1)->dump()
ImplicitCastExpr 0x1463b528 'const volatile void *' <BitCast>
`-ImplicitCastExpr 0x1463b510 'char *' <ArrayToPointerDecay>
  `-StringLiteral 0x1463b440 'char[7]' lvalue "string"
```

I suppose it shouldn't do "IgnoreImpCasts", and instead only ignore a single level of BitCast ImplicitCastExpr?

https://github.com/llvm/llvm-project/pull/99340