[CMake] Set LLVM_PATH in the runtimes build

This avoids using llvm-config for inferring various paths within the
runtimes build. We also set LLVM_INCLUDE_DIR variable that's used by
these builds and move assignment of LLVM_BINARY_DIR and LLVM_LIBRARY_DIR
to the same location for consistency.

Differential Revision: https://reviews.llvm.org/D62637

llvm-svn: 362047