lost expected assignment after "const" error

[JeffBezanson]

julia> const x
ERROR: `body` expression must terminate in `return`. Use `block` instead.

julia> const x::Int
# allowed

These both used to give the "expected assignment" error. This is because we need to parse const x inside struct definitions, and const x is a valid IR form but not generally allowed as input syntax. We need to either fix the error, or decide to allow const x (which declares x constant without giving it a value, which is a state we didn't want to be observable for too long).

[JeffBezanson]

If we fix this in lowering, there's a chance it could be breaking. We currently allow lowering code containing Expr(:const, :x). If we need to keep allowing that, the only way to give an error is during parsing, so we would need to selectively disable the error inside structs.