[lldb/CommandInterpreter] Add CommandInterpreterRunResult (NFC)

This patch adds a new class CommandInterpreterRunResult which will be
backing the SBCommandInterpreterRunResult. It keeps track of the number
of errors as well as the result which is an enum, as proposed by Pavel
in D79120. The command interpreter now populates the results directly,
instead of its own member variables.

Differential revision: https://reviews.llvm.org/D79209