[llvm] [ProfileData] Support merging MD5-based ProfileSymbolList (PR #226594)

[Snehasish Kumar via llvm-commits]

snehasish wrote:

Could we also handle merging MD5-based symbol lists when `-md5-prof-sym-list` is omitted? For example:

```sh
llvm-profdata merge -sample -extbinary a.md5.prof b.md5.prof -o merged.prof
```

With this change, merging succeeds and produces an MD5-backed `WriterList`, but the writer still selects the string-based output path unless the flag is explicitly supplied. This hits the assertion in `writeStringBasedProfileSymbolListSection()` in assertion-enabled builds. With assertions disabled, `ProfileSymbolList::write()` serializes the empty `Syms` set, silently dropping the symbol list.

Could we automatically preserve MD5 encoding when the merged list is MD5-backed?

https://github.com/llvm/llvm-project/pull/226594