Tag the 10.0.0 version bump.

The tag name, llvmorg-10-init, signifies that this is the initial commit
with major version number 10.

With this tag in place, git-describe can be used to generate nicer
names for commits on master, for example:

  $ git describe <hash>
  llvmorg-10-init-<number>-g<hash>

where <number> is the number of commits since this tag.

The plan is to add this kind of tag together with future version bumps.