Update GettingStarted guide to recommend that people use the new

official Git repository.

Remove the directions for using git-svn, and demote the prominence of
the svn instructions.

Also, fix a few other issues while I'm in there:

* Mention LLVM_ENABLE_PROJECTS more.
* Getting started doesn't need to mention test-suite, but should
  mention clang and the other projects.
* Remove mentions of "configure", since that's long gone.

I've also adjusted a few other mentions of svn to point to github, but
have not done so comprehensively.

Differential Revision: https://reviews.llvm.org/D56654

llvm-svn: 351130