Subzero: Fix some build problems against LLVM trunk.

This allows Subzero to build through LLVM e8516587a2604386a8faaab28c410663c2ec884c.

1. Mirror an internal change to the llvm::cl implementation.

2. Something changed in llvm::format() that now requires an explicit conversion from RegNumT.

BUG= none
R=kschimpf@google.com

Review-Url: https://codereview.chromium.org/2602713002 .