With this commit the macro `memnew_placement` uses the standard memory
placement syntax: `new (mem) TheClass()`, and removes the outdated and
not used syntax:
```
_ALWAYS_INLINE_ void *operator new(size_t p_size, void *p_pointer, size_t check, const char *p_description) {
```
Thanks to this change, the function `memnew_placement` call is compatible with
any class, and can also initialize classes with non-empty constructor:
```
// This is valid, like before.
memnew_placement(mem, Variant);
// This works too:
memnew_placement(mem, Variant(123));
```
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