[analyzer] Fix an bug where statically linked, but not registered checkers weren't recognized

My last patch, D56989, moved the validation of whether a checker exists into
its constructor, but we do support statically linked (and non-plugin) checkers
that were do not have an entry in Checkers.td. However, the handling of this
happens after the creation of the CheckerRegistry object.

This patch fixes this bug by moving even this functionality into
CheckerRegistry's constructor.

llvm-svn: 352284