Rollup merge of #45784 - harpocrates:fix/print-parens-cast-lt, r=kennytm

Pretty print parens around casts on the LHS of `<`/`<<`

When pretty printing a cast expression occuring on the LHS of a `<` or `<<` expression, we should add parens around the cast. Otherwise, the `<`/`<<` gets interpreted as the beginning of the generics for the type on the RHS of the cast.

Consider:

    $ cat parens_cast.rs
    macro_rules! negative {
        ($e:expr) => { $e < 0 }
    }

    fn main() {
        negative!(1 as i32);
    }

Before this PR, the output of the following is not valid Rust:

    $ rustc -Z unstable-options --pretty=expanded parens_cast.rs
    #![feature(prelude_import)]
    #![no_std]
    #[prelude_import]
    use std::prelude::v1::*;
    #[macro_use]
    extern crate std as std;
    macro_rules! negative(( $ e : expr ) => { $ e < 0 });

    fn main() { 1 as i32 < 0; }

After this PR, the output of the following is valid Rust:

    $ rustc -Z unstable-options --pretty=expanded parens_cast.rs
    #![feature(prelude_import)]
    #![no_std]
    #[prelude_import]
    use std::prelude::v1::*;
    #[macro_use]
    extern crate std as std;
    macro_rules! negative(( $ e : expr ) => { $ e < 0 });

    fn main() { (1 as i32) < 0; }

I've gone through several README/wiki style documents but I'm still not sure where to test this though. I'm not even sure if this sort of thing is tested...

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