[CMake] Rename cxx_headers back to cxx-headers.

r334477 renamed the cxx-headers target to cxx_headers, but various
pieces sort-of expect the target names to match the component (e.g.,
LLVM_DISTRIBUTION_COMPONENTS in the various bootstrap caches, which, via
some magic foreign to me, seems to expect cxx-headers,
install-cxx-headers, and install-cxx-headers-stripped to exist.)

Revert back to cxx-headers.

Differential Revision: https://reviews.llvm.org/D48701

llvm-svn: 335899