[llvm] [CodeGen] Refactor `warn()` to use StringRef (PR #127537)

[Benjamin Maxwell via llvm-commits]

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@@ -204,7 +204,7 @@ Expected<Header> Header::readFromBuffer(const unsigned char *Curr) {
 
 namespace cgdata {
 
-void warn(Twine Message, std::string Whence, std::string Hint) {
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MacDue wrote:

nit: In the `warn` function below this `void warn(Error E, StringRef Whence)` it calls this `warn()` overload via `warn(IPE.message(), Whence.str(), "");`, I think `Whence.str()` can be replaced with just `Whence`. `Whence.str()` is pointlessly constructing a new `std::string`.

https://github.com/llvm/llvm-project/pull/127537