Eliminate boilerplate of having each test manually assign to `mydir` by calling
`compute_mydir` in lldbtest.py.
Differential Revision: https://reviews.llvm.org/D128077
On the buildbot long and int have the same size but long and long long don't,
so the bug where we find the first type by size will produce a different error.
Make the test dynamic based on int/long/long long size to fix the bot.
