Auto merge of #24683 - P1start:help-suggestions, r=nrc

This PR uses the inline error suggestions introduced in #24242 to modify a few existing `help` messages. The new errors look like this:

    foobar.rs:5:12: 5:25 error: expected a path on the left-hand side of `+`,
                                not `&'static Copy` [E0178]
    foobar.rs:5     let x: &'static Copy + 'static;
                           ^~~~~~~~~~~~~
    foobar.rs:5:12: 5:35 help: try adding parentheses (per RFC 438):
    foobar.rs:      let x: &'static (Copy + 'static);


    foobar.rs:2:13: 2:23 error: cast to unsized type: `&_` as `core::marker::Copy`
    foobar.rs:2     let x = &1 as Copy;
                            ^~~~~~~~~~
    foobar.rs:2:19: 2:23 help: try casting to a reference instead:
    foobar.rs:      let x = &1 as &Copy;


    foobar.rs:7:24: 7:25 error: expected expression, found `;`
    foobar.rs:7     let x = box (1 + 1);
                                       ^
    foobar.rs:7:13: 7:16 help: try using `box()` instead:
    foobar.rs:      let x = box() (1 + 1);

This also modifies compiletest to give the ability to directly test suggestions given by error messages.

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