Add a placeholder for the test-suite-externals directory (#186)

When setting up external sources within llvm-test-suite, that can
either be done by checking out those sources in the
test-suite-externals directory, or by setting a CMake variable to
point at the source directory elsewhere.

As the git repo itself doesn't contain any files in the
test-suite-externals directory, no such directory exists in users
checkouts, unless they manually create it (which feels brittle
and can be prone to mistypings etc).

Therefore, add a placeholder dummy file to force git to create
the directory, making it more obvious to users where they can place
external sources. The directory is ignored via .gitignore, so files
or directories in this tree don't get added accidentally.