[clang][dataflow] Implement transferBranch

This patch introduces `transferBranch`, which Applies the analysis
transfer function for a given edge from a CFG block of a conditional
statement.

RFC:
https://discourse.llvm.org/t/rfc-clang-dataflow-signanalysis-edgetransfer-branchtransfer/65220

Differential Revision: https://reviews.llvm.org/D133698