[clang-tidy] enhance readability-else-after-return

`readability-else-after-return` only warns about `return` calls, but LLVM Coding
Standars stat that `throw`, `continue`, `goto`, etc after `return` calls are
bad, too.

Reviwers: alexfh, aaron.ballman

Differential Revision: https://reviews.llvm.org/D23265

llvm-svn: 278257