CMake: polish the Windows packaging rules

This tweaks the CMake rules for building an installation package on Windows:
 - Sets license file (otherwise nsis shows an ugly default)
 - Adds LLVM logo
 - Shows "do you want to add this to the system path" dialog.

Differential Revision: http://llvm-reviews.chandlerc.com/D1414

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188509 91177308-0d34-0410-b5e6-96231b3b80d8