Show better warning for trying to cast non-u8 scalar to char

[Manishearth]

fn main() {
    const XYZ: char = 0xEF8888 as char;
    println!("{}", XYZ);
}

Will warn "literal is out of range for u8" because u8 is the only as char cast that is acceptable.

We should show a better warning and suggest the \u syntax instead.

[estebank]

In the following code:

rust/src/librustc_lint/types.rs

Lines 165 to 182 in 442b7bd

	ty::TyUint(t) => {
	let uint_type = if let ast::UintTy::Us = t {
	cx.sess().target.usize_ty
	} else {
	t
	};
	let (min, max) = uint_ty_range(uint_type);
	let lit_val: u128 = match lit.node {
	// _v is u8, within range by definition
	ast::LitKind::Byte(_v) => return,
	ast::LitKind::Int(v, _) => v,
	_ => bug!(),
	};
	if lit_val < min || lit_val > max {
	cx.span_lint(OVERFLOWING_LITERALS,
	e.span,
	&format!("literal out of range for {:?}", t));
	}

check if e.hir_id parent expr is a cast to char and emit a help explaining that only u8 can be casted to char and a suggestion to write const XYZ: char = '\u{EF8888}'; instead.