[lldb] Move ObjectFileJIT to lldbExpression

In the spirit of not having lldbExpression rely on plugins, this move
makes the most sense. ObjectFileJIT is not really a "plugin" in the
sense that without it, expression evaluation doesn't work at all. This
is different than something like ObjectFileELF where lldb can still
technically debug non-ELF targets without it. For that reason, moving
ObjectFileJIT into Expression where it will be used in conjunction with
IRExecutionUnit makes the most sense.

Differential Revision: https://reviews.llvm.org/D147084