[clang-diff] Refactor stop-after command-line flag
Summary:
Rename stop-after to stop-diff-after. When building LLVM with
-DLLVM_BUILD_LLVM_DYLIB=ON, stop-after collides with the stop-after
already present in LLVM.
Reviewers: johannes, arphaman
Subscribers: klimek, aheejin, cfe-commits
Differential Revision: https://reviews.llvm.org/D36989