Do not use generated variable names in type errors

[tsandall]

For example:

$ docker run -it --rm openpolicyagent/opa:0.10.7
OPA 0.10.7 (commit 0f39c27, built at 2019-04-09T00:29:33Z)

Run 'help' to see a list of commands.

> 
> 
> x := {"foo": 1}; x.bar == 1
1 error occurred: 1:18: rego_type_error: undefined ref: __local0__.bar
	__local0__.bar
	           ^
	           have: "bar"
	           want (one of): ["foo"]

In this case, the error refers to __local0__ because x is assigned with := and the compiler has rewritten the variable name.

The error should refer to x.