CMake: Some changes to package version names:

- Allow overriding PACKAGE_VERSION from the command-line
- Use PACKAGE_VERSION to set CPACK_PACKAGE_VERSION (used by the Win installer)
- Don't include the version number in the CPack install dir or registry key.

Differential revision: http://llvm-reviews.chandlerc.com/D2245

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195379 91177308-0d34-0410-b5e6-96231b3b80d8