[lldb] Remove use of ConstString from Args::GetShellSafeArgument

[lldb] Remove use of ConstString from Args::GetShellSafeArgument

Having the names of various shells in ConstString's StringPool is not
really necessary, especially if they are otherwise not going to be there
in the first place. For example, if the person debugging uses bash on
their system, the `shell` parameter will have its `m_filename` set to a
ConstString containing "bash". However, fish, tcsh, zsh, and sh will
probably never be used and are just taking up space in the StringPool.

Differential Revision: https://reviews.llvm.org/D148402