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
b95150418f