[analyzer] Overhauling of the checker registration mechanism.

-Checkers will be defined in the tablegen file 'Checkers.td'.
-Apart from checkers, we can define checker "packages" that will contain a collection of checkers.
-Checkers can be enabled with -analyzer-checker=<name> and disabled with -analyzer-disable-checker=<name> e.g:
	Enable checkers from 'cocoa' and 'corefoundation' packages except the self-initialization checker:
	-analyzer-checker=cocoa -analyzer-checker=corefoundation -analyzer-disable-checker=cocoa.SelfInit
-Introduces CheckerManager and CheckerProvider. CheckerProviders get the set of checker names to enable/disable and
 register them with the CheckerManager which will be the entry point for all checker-related functionality.

Currently only the self-initialization checker takes advantage of the new mechanism.

llvm-svn: 125503