Adjust documentation for git migration.

This fixes most references to the paths:
 llvm.org/svn/
 llvm.org/git/
 llvm.org/viewvc/
 github.com/llvm-mirror/
 github.com/llvm-project/
 reviews.llvm.org/diffusion/

to instead point to https://github.com/llvm/llvm-project.

This is *not* a trivial substitution, because additionally, all the
checkout instructions had to be migrated to instruct users on how to
use the monorepo layout, setting LLVM_ENABLE_PROJECTS instead of
checking out various projects into various subdirectories.

I've attempted to not change any scripts here, only documentation. The
scripts will have to be addressed separately.

Additionally, I've deleted one document which appeared to be outdated
and unneeded:
  lldb/docs/building-with-debug-llvm.txt

Differential Revision: https://reviews.llvm.org/D57330

llvm-svn: 352514
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 5d71fc5d7b5ffe2323418a09db6eddaf84d6c662
diff --git a/docs/index.rst b/docs/index.rst
index fddf74b..80c2a8d 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -190,5 +190,4 @@
 * `LLVM Bugzilla <https://bugs.llvm.org/>`_
 * `libcxx-commits Mailing List`_
 * `libcxx-dev Mailing List`_
-* `Browse libc++ -- SVN <http://llvm.org/svn/llvm-project/libcxx/trunk/>`_
-* `Browse libc++ -- ViewVC <http://llvm.org/viewvc/llvm-project/libcxx/trunk/>`_
+* `Browse libc++ Sources <https://github.com/llvm/llvm-project/tree/master/libcxx/>`_