[CMake] Fix the setting of LIBCXX_HEADER_DIR in standalone build

This is an alternative approach to r337727 which broke the build
because libc++ headers were copied into the location outside of
directories used by Clang. This change sets LIBCXX_HEADER_DIR to
different values depending on whether libc++ is being built as
part of LLVM w/ per-target multiarch runtime, LLVM or standalone.

Differential Revision: https://reviews.llvm.org/D49711

llvm-svn: 337833