[clang] [llvm] [sancov] Add -fsanitize-coverage=dataflow-args,dataflow-ret (PR #201410)

[Marco Elver via cfe-commits]

melver wrote:

This is complex enough that it needs LLVM RFC: https://llvm.org/docs/RFCProcess.html

cc @ramosian-glider 

https://github.com/llvm/llvm-project/pull/201410