disallow ccall and cglobal as variable names

[JeffBezanson]

These used to be normal identifiers but over time have become quasi-syntax. As a result you get behavior like this:

julia> ccall(x) = x
ccall (generic function with 1 method)

julia> ccall(2)
ERROR: syntax: too few arguments to ccall

This change prevents that from happening by disallowing assigning to or otherwise creating a variable with these names.

[vtjnash]

Makes sense for ccall, since I think we're going to keep the current behavior (due to the shear inertia of the existing code). For cglobal, it's not clear to my why we should be treating this as special syntax. I'm pretty sure we can easily either make this a perfectly normal function, or make it a perfectly normal macro – I think we just need to decide what semantics it's suppose to guarantee (is it syntax or optimization?).

[JeffBezanson]

There are some cases where we treat cglobal specially, e.g. #26504 and linearization. If it's possible to reliably optimize for constant arguments even when it's a normal function, then that would be ideal. Making it a macro would be ok if that's not possible.