bug in method signatures with nested `where` inside variable bounds

[JeffBezanson]

julia> function f(x::Pair{A,B}) where {A, B<:(Vector{A} where A)}
       end
ERROR: syntax: invalid variable expression in "where"

This happens because of the odd way method signatures are lowered and the name collision with the inner and outer A; it's not a problem for where expressions generally:

julia> Pair{A,B} where {A, B<:(Vector{A} where A)}
Pair{A,B} where B<:(Array{A,1} where A) where A