This commit makes it possible to obtain pointers to `extern` variables
at comptime.
- `ir_get_var_ptr` employs several checks to determine if the given
variable is eligible for obtaining its pointer at comptime. This
commit alters these checks to consider `extern` variables, which have
runtime values, as eligible.
- After this change, it's now possible for `render_const_val` to be
called for `extern` variables. This commit modifies
`render_const_val` to suppress the value generation for `extern`
variables.
- `do_code_gen` now creates `ZigValue::llvm_global` of `extern`
variables before iterating through module-level variables so that
other module-level variables can refer to them.
This solution is incomplete since there are several cases still
failing:
- `global_var.array[n..m]`
- `&global_var.array[i]`
- `&global_var.inner_struct.value`
- `&global_array[i]`
Closes#5349
