cmake + xcode: prevent gtests from using includes from project root

Summary:
At present, several gtests in the lldb open source codebase are using
#include statements rooted at $(SOURCE_ROOT)/${LLDB_PROJECT_ROOT}.

This patch cleans up this directory/include structure for both CMake and
Xcode build systems.

rdar://problem/33835795

Reviewers: zturner, jingham, beanz

Reviewed By: beanz

Subscribers: emaste, lldb-commits, mgorny

Differential Revision: https://reviews.llvm.org/D36598

llvm-svn: 314849