Update Clang Windows getting started docs

Summary:
- Update the example VS project generation to use VS2017.
- Add docs for generating ninja build files, since they are popular.
- Remove reference to "make update" which no longer exists. Mention the
  monorepo instead.
- Try to explain gnuwin32/coreutils requirements better.
- Use https:// links where possible

Reviewers: zturner, STL_MSFT

Subscribers: jfb, cfe-commits

Differential Revision: https://reviews.llvm.org/D52843

llvm-svn: 343809