[clang] [llvm] Replace autoconversion with TextEncodingConverter converters (PR #206519)

[Hubert Tong via cfe-commits]

================
@@ -114,6 +115,31 @@ inline ErrorOr<bool> needConversion(const Twine &FileName, const int FD = -1) {
   return false;
 }
 
+inline ErrorOr<SmallString<32>>
+getEncodingNameFromFileTag(const Twine &FileName, const int FD = -1) {
+#ifdef __MVS__
+  ErrorOr<__ccsid_t> TagOrErr = getzOSFileTag(FileName, FD);
+  if (!TagOrErr)
+    return TagOrErr.getError();
+
+  __ccsid_t Tag = *TagOrErr;
+  if (Tag == 0)
+    return SmallString<32>(); // Return empty string for no tag
+
+  if (Tag == 1208)
+    return SmallString<32>("utf-8");
+
+  if (Tag == 1047)
+    return SmallString<32>("ibm-1047");
+
+  SmallString<32> Result;
+  raw_svector_ostream(Result) << Tag;
+  return Result;
----------------
hubert-reinterpretcast wrote:

> * UCS2, UTF-16, UTF32 should be mapped to empty string - I don't think we have any desire to support double width input encodings.

Considering that we are talking about the source encoding (not the literal encoding), this seems a bit arbitrary to me. I think this needs to be a point of discussion on the RFC (please bring it up if I forget).

https://github.com/llvm/llvm-project/pull/206519