Avoid using `..` in paths for include dirs.

Summary:
This breaks when using a symlink from llvm/tools/lldb to the lldb source
tree, instead of cloning directly as a child. With this change, we can
build properly, even when using links.

Reviewers: dawn, brucem, zturner

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D14089

llvm-svn: 251530