[lldb/Target] Add custom interpreter option to `platform shell`

This patch adds the ability to use a custom interpreter with the
`platform shell` command. If the user set the `-s|--shell` option
with the path to a binary, lldb passes it down to the platform's
`RunShellProcess` method and set it as the shell to use in
`ProcessLaunchInfo to run commands.

Note that not all the Platforms support running shell commands with
custom interpreters (i.e. RemoteGDBServer is only expected to use the
default shell).

This patch also makes some refactoring and cleanups, like swapping
CString for StringRef when possible and updating `SBPlatformShellCommand`
with new methods and a new constructor.

rdar://67759256

Differential Revision: https://reviews.llvm.org/D86667



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>