Don't create links in llvm-prepare

It's a good idea to keep the knowledge about links in as few places as possible,
and we already have a script that deals with links in particular: llvm-projs.

With this change, building llvm after running llvm-prepare will by default build
only llvm (without clang, compiler-rt etc). In order to build other projects as
well, it will be necessary to run llvm-projs with the corresponding args.

This commit also contains a nit: extracting the repo paths into variables
(gitroot and svnroot).

Change-Id: I85719b5ba4866b74e9983584eeabc7e79016d1c9
1 file changed