[lldb] Compute fully qualified command names in FindCommandsForApropos

Fixes incomplete command names in `apropos` results.

The full command names given by `apropos` have come from command name string
literals given to `CommandObject` constructors. For most commands, this has
been accurate, but some commands have incorrect strings. This results in
`apropos` output that doesn't tell the user the full command name they might
want learn more about. These strings can be fixed.

There's a seperate issue that can't be fixed as easily: plugin commands. With
the way they're implemented, plugin commands have to exclude the root command
from their command name string. To illustrate, the `language objc` subcommand
has to set its command name string to "objc", which results in apropos printing
results as `objc ...` instead of `language objc ...`.

To fix both of these issues, this commit changes `FindCommandsForApropos` to
derive the fully qualified command name using the keys of subcommand maps.

Differential Revision: https://reviews.llvm.org/D116491

(cherry picked from commit b3bfd595)