[clang][dataflow] Remove obsolete FIXME

The FIXME is no longer relevant as ControlFlowContext centralizes the
construction of the CFG.

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/D117563