[clang] [llvm] Enable fexec-charset option (PR #138895)

[Sergei Barannikov via cfe-commits]

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@@ -367,6 +385,20 @@ static unsigned ProcessCharEscape(const char *ThisTokBegin,
     HadError = true;
   }
 
+  if (!HadError && EvalMethod != StringLiteralEvalMethod::Unevaluated &&
+      Transcode && Converter) {
+    // Invalid escapes are written as '?' and then translated.
+    char ByteChar = Invalid ? '?' : ResultChar;
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s-barannikov wrote:

Assert that ResultChar fits into `char` type?

https://github.com/llvm/llvm-project/pull/138895