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.

AK/Base64.cpp

@@ -87,10 +87,7 @@ ByteBuffer decode_base64(const StringView& input)
  output.append(out2);
  }
 
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
  return ByteBuffer::copy(output.data(), output.size());
-#pragma GCC diagnostic pop
 }
 
 String encode_base64(ReadonlyBytes input)

CMakeLists.txt

@@ -218,6 +218,7 @@ add_compile_options(-Wno-unknown-warning-option)
 add_compile_options(-Wundef)
 add_compile_options(-Wunused)
 add_compile_options(-Wwrite-strings)
+add_compile_options(-Wno-maybe-uninitialized)
 
 add_compile_options(-ffile-prefix-map=${CMAKE_SOURCE_DIR}=.)
 add_compile_options(-fno-exceptions)

Kernel/Syscall.cpp

@@ -120,15 +120,7 @@ KResultOr<FlatPtr> handle(RegisterState& regs, FlatPtr function, FlatPtr arg1, F
  return ENOSYS;
  }
 
- // This appears to be a bogus warning, as s_syscall_table is always
- // initialized, and the index (function) is always bounded.
- // TODO: Figure out how to avoid the suppression.
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
-
  return (process.*(s_syscall_table[function]))(arg1, arg2, arg3);
-
-#pragma GCC diagnostic pop
 }
 
 }

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;
  }

Userland/Utilities/crash.cpp

@@ -118,10 +118,7 @@ int main(int argc, char** argv)
  if (!uninitialized_memory)
  return Crash::Failure::UnexpectedError;
 
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
  [[maybe_unused]] volatile auto x = uninitialized_memory[0][0];
-#pragma GCC diagnostic pop
  return Crash::Failure::DidNotCrash;
  }).run(run_type);
  }
@@ -144,10 +141,7 @@ int main(int argc, char** argv)
  if (!uninitialized_memory)
  return Crash::Failure::UnexpectedError;
 
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
  uninitialized_memory[4][0] = 1;
-#pragma GCC diagnostic pop
  return Crash::Failure::DidNotCrash;
  }).run(run_type);
  }