Adding install targets for individual LLVM tools and libraries.

Summary:
* add_llvm_tool and add_llvm_library now add install-${name} targets to install specific components
* added installhdrs target to install just the LLVM headers
* The above changes only apply for single-configuration generators (Ninja, Makefiles...), not for multi-configuration generators (Visual Studio, Xcode...)

Reviewers: pete

Reviewed By: pete

Subscribers: pete, llvm-commits

Differential Revision: http://reviews.llvm.org/D7619

llvm-svn: 229727