Clarify error on all-underscore field name that isn't _.

[goretkin]

julia> struct Good{T}
       _::T
       end

# (no error)

julia> struct Bad{T}
       __::T
       end
ERROR: syntax: all-underscore identifier used as rvalue

For what it's worth, I like this behavior. I just think the error message should be improved since, 1. it doesn't seem like an rvalue to me (maybe that's my fault, but see below), and 2. you can have an all-underscore field name so long as it's _.

Regarding the term rvalue:

julia> mutable struct GoodM{T}
       _::T
       end

julia> GoodM(1)
GoodM{Int64}(1)

julia> GoodM(1)._
1

julia> GoodM(1)._ = 3 # feels like it's an lvalue!
3

[kimikage]

cf. #36865