Fix an issue that Duncan discovered on a specific (no longer current)
version of Ubuntu. It has a very broken multiarch configuration, and so
we need special logic to handle it correctly. Fixing and testing this
uncovered a few other trivial issues with the logic that are fixed as
well.

I added tests to cover this as it is hard to notice if you install
recent versions of the OS.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@144165 91177308-0d34-0410-b5e6-96231b3b80d8
14 files changed