[analyzer] Supply all checkers with a shouldRegister function

Introduce the boolean ento::shouldRegister##CHECKERNAME(const LangOptions &LO)
function very similarly to ento::register##CHECKERNAME. This will force every
checker to implement this function, but maybe it isn't that bad: I saw a lot of
ObjC or C++ specific checkers that should probably not register themselves based
on some LangOptions (mine too), but they do anyways.

A big benefit of this is that all registry functions now register their checker,
once it is called, registration is guaranteed.

This patch is a part of a greater effort to reinvent checker registration, more
info here: D54438#1315953

Differential Revision: https://reviews.llvm.org/D55424

llvm-svn: 352277

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