[clang] Fix suppressing diagnostics for uninitialized variables (#148336)
When one kind of diagnostics is disabled, this should not preclude other
diagnostics from displaying, even if they have lower priority. For
example, this should print a warning about passing an uninitialized
variable as a const reference:
```
> cat test.cpp
void foo(const int &);
int f(bool a) {
int v;
if (a) {
foo(v);
v = 5;
}
return v;
}
> clang test.cpp -fsyntax-only -Wuninitialized -Wno-sometimes-uninitialized
```