[libc++] Fix compiler-rt build by copying libc++ headers to <build>/include
This commit should really be named "Workaround external projects depending
on libc++ build system implementation details". It seems that the compiler-rt
build (and perhaps other projects) is relying on the fact that we copy libc++
and libc++abi headers to <build-root>/include/c++/v1. This was changed
by 5d796645, which moved the headers to <build-root>/projects/libcxx/include/c++/v1
and broke the compiler-rt build.
I'm committing this workaround to fix the compiler-rt build, but we should
remove reliance on implementation details like that. The correct way to
setup the compiler-rt build would be to "link" against the cxx-headers
target in CMake, or to run install-cxx-headers using an appropriate
installation prefix, and then manually add a -I path to that location.