[Clang] Control-flow analysis around expansion statements could be better

Concretely, consider (https://godbolt.org/z/e75Kx1WMr)

int f() {
    template for (int x : {1}) {
        return 4;
    }
}

which currently results in a warning

<source>:5:1: warning: non-void function does not return a value [-Wreturn-type]
    5 | }
      | ^

despite the fact that we do in fact always return here, as the expansion statement effectively expands to

int f() {
    {
        {
            int x = 1;
            return 4;
        }
    }
}

which we don’t warn about. Presumably, the fix is that we should probably be walking the CXXExpansionStmtInstantiation of an expansion statement rather than the pattern.

Start with the Godbolt example linked in the issue and trace Clang's control-flow analysis for the expansion statement. Compare how analysis treats the CXXExpansionStmtInstantiation with how it treats the pattern. Done means the shown function is recognized as always returning and no longer produces the -Wreturn-type warning.