Add CMAKE_CFG_INTDIR as part of the include path for the default test compiler.

Summary:
This is a small change towards running the LLDB unit tests on Windows (still WIP).

Builds that use make do not point to specific configurations. However, builds with multiple configurations such as Visual Studio on Windows need the configuration (release/debug/etc) as part of the path so the binaries produced by clang are placed in the correct directory for the configuration.

Reviewers: zturner, lldb-commits, labath

Reviewed By: labath

Subscribers: labath, mgorny, hintonda, llvm-commits

Differential Revision: https://reviews.llvm.org/D42990

llvm-svn: 324555