[analyzer] NFC: Introduce sub-classes for path-sensitive and basic reports.

Checkers are now required to specify whether they're creating a
path-sensitive report or a path-insensitive report by constructing an
object of the respective type.

This makes BugReporter more independent from the rest of the Static Analyzer
because all Analyzer-specific code is now in sub-classes.

Differential Revision: https://reviews.llvm.org/D66572

llvm-svn: 371450