[analyzer] Replace boolean IsSink parameters with 'generateSink' methods.

Generating a sink is significantly different behavior from generating a
normal node, and a simple boolean parameter can be rather opaque. Per
offline discussion with Anna, adding new generation methods is the
clearest way to communicate intent.

No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@162215 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed