lib/Header: Fix Visual Studio builds try #2

Summary:
This is a follow up to r355253 and a better fix than the first attempt
which was r359257.

We can't install anything from ${CMAKE_CFG_INTDIR}, because this value
is only defined at build time, but we still must make sure to copy the
headers into ${CMAKE_CFG_INTDIR}/lib/clang/$VERSION/include, because the lit
tests look for headers there.  So for this fix we revert to the
old behavior of copying the headers to ${CMAKE_CFG_INTDIR}/lib/clang/$VERSION/include
during the build and then installing them from the source tree.

Reviewers: smeenai, vzakhari, phosek

Reviewed By: smeenai, vzakhari

Subscribers: mgorny, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61220

llvm-svn: 359654