Error (set/union) improvements

[FnControlOption]

Add completions for merged error sets

Closes #1130

Implement type resolution for error union payloads

Closes #1245

Enables hover and type resolution for val and err in the following code:

const std = @import("std");

pub fn main() void {
    foo() catch {};
    baz(true);
    baz(false);
}

const Error = error{Foo};

fn foo() Error!void {
    errdefer |err| {
        std.log.err("errdefer - {}", .{@TypeOf(err)});
    }
    return Error.Foo;
}

fn bar(return_error: bool) Error!?i32 {
    if (return_error)
        return Error.Foo;
    return 123;
}

fn baz(comptime return_error: bool) void {
    std.log.info("catch - {}", .{comptime blk: {
        const val = bar(return_error) catch |err| {
            break :blk @TypeOf(err);
        };
        break :blk @TypeOf(val);
    }});

    std.log.info("if - {}", .{comptime blk: {
        if (bar(return_error)) |val| {
            break :blk @TypeOf(val);
        } else |err| {
            break :blk @TypeOf(err);
        }
    }});

    std.log.info("while - {}", .{comptime blk: {
        while (bar(return_error)) |val| {
            break :blk @TypeOf(val);
        } else |err| {
            break :blk @TypeOf(err);
        }
    }});
}

Todo

• Infer errdefer error type based on the enclosing function's return type
• Investigate why autocompletion doesn't work for error union capture in empty while block