Handle `let _ = x` in coercing closures to FnPtr

[arora-aman]

Consider the following closure:

let x;

let c = || {
    let _ = x;
};

let f: fn() = c; 

When capture_disjoint_fields is enabled, this closure doesn't capture x, implying c can be coerced to an FnPtr.

However, the code that checks if coercion can happen depends on upvars_mentioned which are computed before the capture analysis and would result in an error.

[arora-aman]

Documenting a solution @nikomatsakis has previously proposed:

• When the coercion code runs in Typeck we set an Obligation for the closure to have no captures/ can be coerced to an FnPtr.
• After capture analysis, in trait selection, we ensure that the obligation can be met.

[arora-aman]

After #24