[CMake][runtimes] Set clang-header dependency for builtins

compiler-rt builtins depend on clang headers, but that dependency
wasn't explicitly stated in the build system and we were relying
on the transitive depenendecy via clang. However, when we're
cross-compiling clang, we'll be using host compiler instead and
that depenendecy is missing, breaking the build.

Differential Revision: https://reviews.llvm.org/D58471

llvm-svn: 354524