[clang][dataflow] Make `runDataflowReturnError()` a non-template function.

It turns out this didn't need to be a template at all.

Likewise, change callers to they're non-template functions.

Also, correct / clarify some comments in RecordOps.h.

This is in response to post-commit comments on https://reviews.llvm.org/D153006.

Reviewed By: gribozavr2

Differential Revision: https://reviews.llvm.org/D154339