lib/Header: Simplify CMakeLists.txt

Summary:
Replace cut and pasted code with cmake macros and reduce the number of
install commands.  This fixes an issue where the headers were being
installed twice.

This clean up should also make future modifications easier, like
adding a cmake option to install header files into a custom resource
directory.

Reviewers: chandlerc, smeenai, mgorny, beanz, phosek

Reviewed By: smeenai

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58537

llvm-svn: 355253