Find clang resource directory via *nix-style lookup

Summary:
This patch allows the Darwin build to fall back to to Posix-style lookups for the clang resource directory if the debugger library isn't inside a framework.

The patch also includes a bit of refactoring and cleanup around the *nix resolution of the binary and lib directories to reuse the code instead of duplicating it.

With this patch Darwin builds that don't build a framework only have 3 failing tests on my system (TestExec.py).

Reviewers: zturner, labath, spyffe, tfiala

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D26170

llvm-svn: 285838