[lldb] Fix alias parsing with joined options (#190301)
Fixes a crash with the following alias, which I use for printing the
contents of pointer variables:
```
command alias vp v -P1
```
At some point in the recent-ish past, parsing this alias has started
crashing lldb. The problem is code that assumes the option and its value
are separate. This assumption causes an index past the end of a vector.
This fix changes `FindArgumentIndexForOption`. The function now returns
a pair of indexes, the first index is the option, the second index is
the index of the value. In the case of joined options like `-P1`, the
two indexes are the same.
