[clang][dataflow] Include terminator statements in buildStmtToBasicBlockMap

This will be necessary later when we add support for evaluating logic
expressions such as && and ||.

This is part of the implementation of the dataflow analysis framework.
See "[RFC] A dataflow analysis framework for Clang AST" on cfe-dev.

Reviewed-by: xazax.hun

Differential Revision: https://reviews.llvm.org/D119447