[clang-rename] Merge rename-{at|all} & optimize.

Having both rename-at and rename-all both seems confusing and introduces
unneeded difficulties. Allowing to use both -qualified-name and -offset at once
while performing efficient renamings seems like a feature, too. Maintaining main
function wrappers and custom help becomes redundant while CLI becomes less
confusing.

Reviewers: alexfh

Differential Revision: https://reviews.llvm.org/D24224

llvm-svn: 281456