[clang] [C23] Implement N3018: The constexpr specifier for object definitions (PR #73099)

[Aaron Ballman via cfe-commits]

================
@@ -2487,14 +2487,19 @@ bool VarDecl::mightBeUsableInConstantExpressions(const ASTContext &C) const {
   if (!getType().isConstant(C) || getType().isVolatileQualified())
     return false;
 
-  // In C++, const, non-volatile variables of integral or enumeration types
-  // can be used in constant expressions.
-  if (getType()->isIntegralOrEnumerationType())
+  // In C++, but not in C, const, non-volatile variables of integral or
+  // enumeration types can be used in constant expressions.
+  if (getType()->isIntegralOrEnumerationType() && !Lang.C23)
----------------
AaronBallman wrote:

We might want to relax this in the future because we *do* allow const, non-volatile integer variables in constant expressions in C: https://godbolt.org/z/jc1h49Wox

But I think the code is fine for now because I think the function is not accurate in other ways:
```
  // Function parameters are never usable in constant expressions.
  if (isa<ParmVarDecl>(this))
    return false;
```
which is not quite the case: https://godbolt.org/z/Kdq9xY617 and yet is the case: https://godbolt.org/z/1zEGsbYfn

https://github.com/llvm/llvm-project/pull/73099