[analyzer] Add CheckerManager::getChecker, make sure that a registry function...

[analyzer] Add CheckerManager::getChecker, make sure that a registry function registers no more than 1 checker

This patch effectively fixes the almost decade old checker naming issue.
The solution is to assert when CheckerManager::getChecker is called on an
unregistered checker, and assert when CheckerManager::registerChecker is called
on a checker that is already registered.

Differential Revision: https://reviews.llvm.org/D55429

llvm-svn: 352292