Update Clang Windows getting started docs

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@343809 91177308-0d34-0410-b5e6-96231b3b80d8