Fix Clang install rules to not set permissions on include/

The CMake install(DIRECTORY) command documents that it sets permissions
on directories it is asked to install.  Since the <prefix>/include
directory may not be exclusive to the LLVM/Clang installation, we should
not ask CMake to manage permissions of that directory for us.  Instead,
give only our own include/clang and include/clang-c subdirectories to
the install(DIRECTORY) command.

Fixes PR4500. Patch by Brad King.

llvm-svn: 201076