Format the last statement as an expression

[topecongiro]

Replace ExprType::Statement with ExprType::SubExpression if the statement is the last statement within its enclosing block.

rustfmt/src/expr.rs

Line 585 in 1cea171

format_expr(ex, ExprType::Statement, context, shape).map(|s| s + suffix)

One way to implement this is to add a wrapper type over ast::Stmt which holds the information about whether it's the last statement or not:

struct Statement<'ast> {
    stmt: &'ast ast::Stmt,
    is_last: bool,
}

cc #3614.