Allow users to create aliases for aliases to raw input commands. That probably
sounds convoluted, so here's an example:

```
command alias some-setup env SOMEVAR=SOMEVALUE
```

This an alias based on `env`, which itself is an alias for `_regex-env`.
`_regex-env` is a `command regex` command, which takes raw input.

The above `some-setup` alias fails with:

```
error: Unable to create requested alias.
```

This change allows such aliases to be created. lldb already supports aliases to
aliases for parsed commands.

Differential Revision: https://reviews.llvm.org/D117259