Fix test suite lookup path for LLDB.h

Summary:
When running on Darwin, the test suite assumes a specific directory structure for the build directory. This works for the Xcode project builds, but fails for CMake builds regardless of whether or not you are generating the LLDB framework.

This patch allows the Darwin code path to fall back to the more generic code path used by other platforms in the event that LLDB.h isn't where the test suite expects it.

This allows API tests to run on Darwin when building with CMake with the framework build enabled or disabled.

Reviewers: tfiala, zturner

Subscribers: labath, lldb-commits

Differential Revision: https://reviews.llvm.org/D25488

llvm-svn: 284043