C backend: enumerate all the types in renderType
C backend: enumerate all the types in renderType

Now that we're close to supporting all the types, get rid of the `else`
prong and explicitly list out those types that are not yet implemented.

Thanks @g-w1

andrewrk/zig,zig-lang/zig,zig-lang/zig,andrewrk/zig,zig-lang/zig