change LLVM_VERSION_SUFFIX default from svn to git

Summary:
Sayonara SVN!

Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

Reviewers: tstellar, jyknight, lebedev.ri, smeenai, mgorny, hans, mclow.lists

Reviewed By: mgorny, hans

Subscribers: christof, libcxx-commits, llvm-commits, srhines

Tags: #libc, #llvm

Differential Revision: https://reviews.llvm.org/D70019