We previously allowed these closures to default to (), but be inferred
as other types as well, which means that we will find some expressions
to be ambiguous because we end up finding multiple viable solutions
where there is really only one reasonable solution.
Fixes: rdar://problem/42337247
We parse default argument expressions before building a
FuncDecl, so we might see GenericTypeParamDecls that have
not yet been re-parented to the FuncDecl's context.
Fixes <https://bugs.swift.org/browse/SR-5559>,
<rdar://problem/33539464>.
