[CMake] Make bootstrap and compiler-rt depend on cxx-headers.

Since r334468, we no longer always copy the libcxx headers by virtue of
their presence when cmake runs.

This makes some of the later stages (compiler-rt, and the bootstrapped
stages) depend on them being copied, via the cxx-headers target.

Differential Revision: https://reviews.llvm.org/D48700

llvm-svn: 335898