[Clang] Improve diagnostics for 'placement new' with const storage argument (#144270)

Before this patch, the following code gave misleading diagnostics about
absence of `#include <new>`:
```cpp
#include <new>

struct X { int n; };
int foo() {
  const X cx = {5};
  // error: no matching 'operator new' function for non-allocating placement new expression; include <new>
  (void)new(&cx) X{10};
};
```
Now it gives correct diagnostics about constness of passed argument:
```cpp
#include <new>

struct X { int n; };
int foo() {
  const X cx = {5};
  // error: placement new expression with a const-qualified argument of type 'const X *' is not allowed
  (void)new(&cx) X{10};
};
```

Fixes https://github.com/llvm/llvm-project/issues/143708.

---------

Co-authored-by: Corentin Jabot <corentinjabot@gmail.com>

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