Flags uses of absl::StrCat() to append to a std::string. Suggests absl::StrAppend() should be used instead.

The extra calls cause unnecessary temporary strings to be constructed. Removing them makes the code smaller and faster.

a = absl::StrCat(a, b); // Use absl::StrAppend(&a, b) instead.

Does not diagnose cases where absl::StrCat() is used as a template argument for a functor.