[llvm] [memprof] Add YAML read/write support to llvm-profdata (PR #118915)

[Snehasish Kumar via llvm-commits]

https://github.com/snehasish approved this pull request.

lgtm modulo @teresajohnson's comment - I think a valid YAML file interpreted as an invalid profile would yield an empty indexed profile. So maybe we could add an assertion somewhere that the profile is non-empty?

https://github.com/llvm/llvm-project/pull/118915