[Clang] [CodeGen] Fix codegen bug in constant initialisation in C23 mode (#84981)

Consider the following code:
```c
bool const inf =  (1.0/0.0);
``` 

When trying to emit the initialiser of this variable in C23, we end up
hitting a code path in codegen in `VarDecl::evaluateValueImpl()` where
we check for `IsConstantInitialization && (Ctx.getLangOpts().CPlusPlus
|| Ctx.getLangOpts().C23)`, and if that is the case and we emitted any
notes, constant evaluation fails, and as a result, codegen issues this
error:
```
<source>:1:12: error: cannot compile this static initializer yet
    1 | bool const inf =  (1.0/0.0);
      |        
```

As a fix, only fail in C23 mode if we’re initialising a `constexpr`
variable.

This fixes #84784.

Author: Sirraide

clang/lib/AST/Decl.cpp

@@ -2577,11 +2577,14 @@ APValue *VarDecl::evaluateValueImpl(SmallVectorImpl<PartialDiagnosticAt> &Notes,
   bool Result = Init->EvaluateAsInitializer(Eval->Evaluated, Ctx, this, Notes,
                                             IsConstantInitialization);
 
-  // In C++/C23, this isn't a constant initializer if we produced notes. In that
-  // case, we can't keep the result, because it may only be correct under the
-  // assumption that the initializer is a constant context.
+  // In C++, or in C23 if we're initialising a 'constexpr' variable, this isn't
+  // a constant initializer if we produced notes. In that case, we can't keep
+  // the result, because it may only be correct under the assumption that the
+  // initializer is a constant context.
   if (IsConstantInitialization &&
-      (Ctx.getLangOpts().CPlusPlus || Ctx.getLangOpts().C23) && !Notes.empty())
+      (Ctx.getLangOpts().CPlusPlus ||
+       (isConstexpr() && Ctx.getLangOpts().C23)) &&
+      !Notes.empty())
     Result = false;
 
   // Ensure the computed APValue is cleaned up later if evaluation succeeded,

clang/test/CodeGen/const-init.c

@@ -216,3 +216,6 @@ int PR4517_x2 = PR4517_arrc[PR4517_idx];
 // CHECK: @PR4517_x = global i32 42, align 4
 // CHECK: @PR4517_idx = constant i32 1, align 4
 // CHECK: @PR4517_x2 = global i32 42, align 4
+
+// CHECK: @GH84784_inf = constant i8 1
+_Bool const GH84784_inf = (1.0/0.0);

clang/test/Sema/const-init.c

@@ -0,0 +1,6 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c23 %s
+
+// Division by 0 here is an error iff the variable is 'constexpr'.
+const _Bool inf1 =  (1.0/0.0 == __builtin_inf());
+constexpr _Bool inf2 = (1.0/0.0 == __builtin_inf()); // expected-error {{must be initialized by a constant expression}} expected-note {{division by zero}}
+constexpr _Bool inf3 = __builtin_inf() == __builtin_inf();