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

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

Having both rename-at and rename-all both seems confusing and introduces
unneeded difficulties. After merging rename-at and rename-all maintaining main
function wrappers and custom help becomes redundant while CLI becomes less
confusing.

D24224 (which was the original patch causing buildbot failures) wasn't aware of
bugs caused by passing both -offset and -qualified-name. After D24224 was landed
it caused buildbot failures and therefor I just reverted it.

Two things that make this patch different from D24224 are:

* unittests/clang-rename was deleted, because it is unmaintained and doesn't do
much.
* Passing both `-offset` and `-qualified-name` isn't allowed anymore for the
sake of preventing bugs.

This patch is a trivial enhancement of accepted D24224 revision.

Tested with `ninja check-all`.

Differential Revision: https://reviews.llvm.org/D24567

llvm-svn: 281710