Fix a mutation of `CommandAlias::m_option_args_sp`, which resulted in cases where
aliases would fail to run on second, and subsequent times.
For example, an alias such as:
```
command alias p1 p 1
```
When run the second time, the following error would be reported to the user:
```
error: expression failed to parse:
error: <user expression 1>:1:1: expression is not assignable
-- 1
^ ~
```
To fix this, `CommandAlias::Desugar` now constructs options to a freshly constructed
vector, rather than by appending to the results of `GetOptionArguments`.
rdar://107770836
Differential Revision: https://reviews.llvm.org/D150078
765237779c