Refactor CFG-based warnings in Sema to be run by a worked object called AnalysisBasedWarnings.

This object controls when the warnings are executed, allowing the client code
in Sema to selectively disable warnings as needed.

Centralizing the logic for analysis-based warnings allows us to optimize
when and how they are run.

Along the way, remove the redundant logic for the 'check fall-through' warning
for blocks; now the same logic is used for both blocks and functions.

llvm-svn: 99085