The command is:
```
dump <context> [<ID>|<TYPE><ID>|<TYPE> <ID>|-- <ID>]
TYPE can be "inst", "entity_name", etc.
```
This saves a lot of typing of `SemIR::MakeInstId()` in a debugger, and
allows copy-pasting ids from dump output, as they take the form
`inst33`, etc.
The docs explain that you must use `--local-lldbinit` in the command
line, and include an example of how to run a file_test under lldb from
the command line.
This PR includes `.lldbinit` file and `lldbinit.py` file which set up
our default options, copied from the VSCode launcher.
The instructions include settin the `max-string-summary-length`, and we
include this in the vscode launcher for lldb, as printing `Dump()`
output can easily get truncated otherwise when printing an InstBlockId.
Dana Jansens