[analyzer] [NFC] Split up RetainSummaryManager from RetainCountChecker

ARCMigrator is using code from RetainCountChecker, which is a layering
violation (and it also does it badly, by using a different header, and
then relying on implementation being present in a header file).

This change splits up RetainSummaryManager into a separate library in
lib/Analysis, which can be used independently of a checker.

Differential Revision: https://reviews.llvm.org/D50934

llvm-svn: 340114