[CMake] Copy headers relative to LLVM_BINARY_DIR

Summary: This copy phase is only needed for in-tree builds, so we should be copying to the LLVM build directory's include dir instead of the sub-project include dir.

Reviewers: bogner, EricWF

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D24015

llvm-svn: 280108