[clang][dataflow] Use namespace qualifiers when defining functions.

See

https://llvm.org/docs/CodingStandards.html#use-namespace-qualifiers-to-implement-previously-declared-functions

Thank you to MaskRay for pointing this out on

https://reviews.llvm.org/D153006

Reviewed By: xazax.hun

Differential Revision: https://reviews.llvm.org/D153833