[lldb][modern-type-lookup] Add two basic tests for modern-type-lookup

The story so far: LLDB's modern type lookup mode has no (as in, 0%) test
coverage. It was supposed to be tested by hardcoding the default to 'true' and then running
the normal LLDB tests, but to my knowledge no one is doing that. As a around 130 tests
seem to fail with this mode enabled, we also can't just enable it globally for now.

As we touch the surrounding code all the time and also want to refactor parts of it, we
should be a bit more ambitious with our testing efforts.

So this patch adds two basic tests that enable this mode and do some
basic expression parsing which should hopefully be basic enough to not
break anywhere but still lets us know if this mode works at all (i.e. setting up the
ExternalASTMerger in LLDB, using its basic import functionality to move declarations
around and do some lookups).

llvm-svn: 372869