Add lldb-test.

Add lldb-test.

This is basically a proof-of-concept and starting point for having a
testing-centric tool in LLDB.  I'm sure this leaves a lot of room to be
desired, but this at least allows us to have something to build on.

Right now there is only one command, the `module-sections` command, and I
created this command not because it was particularly special, but
because it addressed an immediate use case and was extremely simple.

Run the tool as `lldb-test module-sections <path-to-object>`.

Feel free to add testing related stuff to your heart's content after
this goes in.  Implementing the commands themselves takes some work, but
once they're there they can be reused without writing any code and
result in very easy to use and maintain tests.

Differential Revision: https://reviews.llvm.org/D40636

llvm-svn: 319504