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.
debuginfo: Make sure that only one LLDB test task is run at a time.

Some older versions of LLDB have problems with multiple instances running in parallel. This commit makes sure we don't do that.
auto merge of #15573 : michaelwoerister/rust/lldb-tests-rebased-09-Jul, r=alexcrichton

This PR adds the LLDB autotests to the debuginfo test suite so I don't have to keep rebasing them locally. They are still disabled by default in `tests.mk`. One of the commits also contains a Python pretty printer which can make LLDB print values with Rust syntax. This was mainly added to deal with output format differences between LLDB versions but you can also use it for your normal LLDB debugging sessions.
```
// The following LLDB commands will load and activate the Rust printers
command script import ./src/etc/lldb_rust_formatters.py
type summary add --no-value --python-function lldb_rust_formatters.print_val -x .* --category Rust
type category enable Rust
```
Expect some rough edges with these, they have not been tested apart from there use in the autotests...