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

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

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

Good point! I've modified `setProfileSymbolList` as follows and updated `profile-symbol-list.test` to merge without `-md5-prof-sym-list`:

```cpp
  void setProfileSymbolList(ProfileSymbolList *PSL) override {
    ProfSymList = PSL;
    if (ProfSymList && ProfSymList->isMD5())
      setUseMD5ProfileSymbolList();
  }
```

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