This patch adds the bare-minimum tests for the LLDB `llvm::ArrayRef`
formatters. Since this would be the first LLVM data fromatter test for
LLDB, I wanted to keep the test itself minimal and mainly set up/agree
on the infrastructure (i.e., CMake machinery, etc.).
The setup mimicks that of GDB. The main differences are:
1. the GDB formatter tests put all the test cases in one monolithic test
file, whereas I'm planning on having one test-file per LLVM container.
2. the GDB formatter tests currently only get run if LLVM was built with
debug-info. Not sure we have any build-bots out there running this
configuration. In this patch we rely on just the debug-info from the
LLVM headers (see latest commit). That way the tests get more
consistently run (at least our public macOS buildbot runs the
cross-project-tests in Release+Asserts).
