Rollup merge of #23310 - michaelwoerister:gdb-std-pp, r=alexcrichton

Rollup merge of #23310 - michaelwoerister:gdb-std-pp, r=alexcrichton

 ```rust
Rust:  let slice: &[i32] = &[0, 1, 2, 3];
GDB:   $1 = &[i32](len: 4) = {0, 1, 2, 3}

Rust:  let vec = vec![4, 5, 6, 7];
GDB:   $2 = Vec<u64>(len: 4, cap: 4) = {4, 5, 6, 7}

Rust:  let str_slice = \"IAMA string slice!\";
GDB:   $3 = \"IAMA string slice!\"

Rust:  let string = \"IAMA string!\".to_string();
GDB:   $4 = \"IAMA string!\"
```
Neat!