[Reflection] Fix a member function so that it actually compiles.

[dcci]

Nobody's using it, so nobody noticed. I plan to use this in the
debugger.

[dcci]

@swift-ci smoke test and merge

[gottesmm]

test?

[dcci]

I can probably change swift-reflection-dump to call this function or we can wait until LLDB uses this or remove the function altogether as nobody uses this. Which one do you prefer?
The reason why I wasn't particularly worried about adding a test is that this function fails to compile without the change, but, yeah, I agree we should considering adding one (or testing this from some consumer).