[CMake] Use cxx-headers as a depedency for C++ headers

We no longer pass CLANG_DEFAULT_CXX_STDLIB to the runtimes build
as it was causing issues so we can no longer use this variable. We
instead use cxx-headers as a dependency whenever this is available
since both XRay and libFuzzer are built as static libraries so this
is sufficient.

Differential Revision: https://reviews.llvm.org/D49346

llvm-svn: 337199