Improve borrow of immutable indexed content with NLL enabled

[estebank]

Follow up on #52788 and produce the following output when enabling NLL:

error[E0596]: cannot borrow immutable indexed content as mutable
  --> $DIR/index-mut-help.rs:21:5
   |
LL |     map["peter"].clear();           //~ ERROR
   |     ^^^^^^^^^^^^ cannot borrow as mutable
   |
   = help: trait `IndexMut` is required to modify indexed content, but it is not implemented for `std::collections::HashMap<&str, std::string::String>`

[matthewjasper]

See also: #52086 (similar issue for Deref)

[nikomatsakis]

Putting this on the Rust 2018 RC milestone; "diagnostics nit".