rustc: Remove `&str` indexing from the language.

Being able to index into the bytes of a string encourages
poor UTF-8 hygiene. To get a view of `&[u8]` from either
a `String` or `&str` slice, use the `as_bytes()` method.

Closes #12710.

[breaking-change]
debuginfo: Add python formatters that allow LLDB to print values with Rust syntax
debuginfo: Inject 'Rust-mode' LLDB prelude into debuginfo test scripts.
debuginfo: Remove atexit()-debugger shutdown from lldb_batchmode.py

The shutdown call would always result in an exception being printed to standard error.
debuginfo: Add LLDB autotests to debuginfo test suite.

This commit adds LLDB autotests to the test suite but does not activate them by default yet.