Add safety check for await inside nosuspend blocks

[mguaypaq]

As discussed in #7972 (specifically, this comment by @SpexGuy), the zig compiler currently (version 0.7.1) doesn't put in a safety check when, inside a nosuspend block, we await an async function which has not yet completed. Instead, a garbage value is silently used as the function's return value.

The compiler should put in a safety check to check whether the awaited function has completed.

pub fn main() void {
    var f = async not_done_yet();

    // This is undefined behavior and should be safety-checked.
    var i = nosuspend await f;

    @import("std").debug.print("garbage: {}\n", .{i});
}

fn not_done_yet() u8 {
    suspend;
    suspend;
    return 2;
}