lib/Header: Fix Visual Studio builds

Summary:
This is a follow up to r355253, which inadvertently broke Visual
Studio builds by trying to copy files from CMAKE_CFG_INTDIR.

See https://reviews.llvm.org/D58537#inline-532492

Reviewers: smeenai, vzakhari, phosek

Reviewed By: smeenai

Subscribers: mgorny, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61054

llvm-svn: 359257