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.
