Global `let` with non-constant initializer crashes in lowering

Description of the bug:

If a global let has an initializer that is non-constant, type-checking accepts but lowering crashes.

What did you do, or what's a simple way to reproduce the bug?

fn F() -> i32;
let a: i32 = F();
fn G() -> i32 { return a; }

What did you expect to happen?

Either this should work (like a global var would), or we should get an error from check that a global let must have a constant initializer.

What actually happened?

We get a CHECK-failure:

CHECK failure at toolchain/lower/function_context.cpp:190: const_id.is_concrete(): Missing value: inst7E000042 {kind: Converted, arg0: inst7E000040, arg1: inst7E000041, type: type(inst7E00002A)} in specific<none> has non-concrete value runtime

Any other information, logs, or outputs that you want to share?

The problem is that we generate a name_ref pointing directly to the value of the let, which is in an unrelated instruction block (the file block) and hence not visible.