This source location will be used to determine whether to add a name lookup
option to exclude macro expansions when the name lookup request is constructed.
Currently, the source location argument is unused.
Also fix a weird latent bug. In lookupConcreteNestedType(), we would
push nullptr onto the concreteDecls vector if the opaque archetype
did not have a nested type with this name. However, this did not
turn out to be a problem, since in this code path we would only
have a single element in this vector, and the later call to
std::min_element() did not end up dereferencing the null pointer.
However this is very dodgy, so tweak the code to prevent this from
happening and add a test case (which already passed anyway).