[DiagnosticInfo] Fix the default DiagnosticSeverity (#120342)

After
ea632e1b34e1

the API call to LLVMContext->emitError(I, Errorstr) default to warning
instead of error.

This cause problems as the API mentioned it is "prefixed with error:".

Author: jsji

llvm/include/llvm/IR/DiagnosticInfo.h

@@ -150,7 +150,7 @@ class DiagnosticInfoGeneric : public DiagnosticInfo {
       : DiagnosticInfo(DK_Generic, Severity), MsgStr(MsgStr) {}
 
   DiagnosticInfoGeneric(const Instruction *I, const Twine &ErrMsg,
-                        DiagnosticSeverity Severity = DS_Warning)
+                        DiagnosticSeverity Severity = DS_Error)
       : DiagnosticInfo(DK_Generic, Severity), MsgStr(ErrMsg), Inst(I) {}
 
   const Twine &getMsgStr() const { return MsgStr; }