Meta: Disable -Wmaybe-uninitialized

It's prone to finding "technically uninitialized but can never happen"
cases, particularly in Optional<T> and Variant<Ts...>.
The general case seems to be that it cannot infer the dependency
between Variant's index (or Optional's boolean state) and a particular
alternative (or Optional's buffer) being untouched.
So it can flag cases like this:
```c++
if (index == StaticIndexForF)
    new (new_buffer) F(move(*bit_cast<F*>(old_buffer)));
```
The code in that branch can _technically_ make a partially initialized
`F`, but that path can never be taken since the buffer holding an
object of type `F` and the condition being true are correlated, and so
will never be taken _unless_ the buffer holds an object of type `F`.

This commit also removed the various 'diagnostic ignored' pragmas used
to work around this warning, as they no longer do anything.

Userland/Libraries/LibGUI/Label.cpp

@@ -142,10 +142,7 @@ void Label::wrap_text()
         case '\t':
         case ' ': {
             if (start.has_value())
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
                 words.append(m_text.substring(start.value(), i - start.value()));
-#pragma GCC diagnostic pop
             start.clear();
             continue;
         }