[NFC] Fix a potential compile error in llvm::to_string(ThinOrFullLTOPhase Phase)

#123994 (comment)

Author: shiltian

llvm/include/llvm/Pass.h

@@ -87,20 +87,7 @@ enum class ThinOrFullLTOPhase {
 };
 
 #ifndef NDEBUG
-constexpr const char *to_string(ThinOrFullLTOPhase Phase) {
-  switch (Phase) {
-  case ThinOrFullLTOPhase::None:
-    return "None";
-  case ThinOrFullLTOPhase::ThinLTOPreLink:
-    return "ThinLTOPreLink";
-  case ThinOrFullLTOPhase::ThinLTOPostLink:
-    return "ThinLTOPostLink";
-  case ThinOrFullLTOPhase::FullLTOPreLink:
-    return "FullLTOPreLink";
-  case ThinOrFullLTOPhase::FullLTOPostLink:
-    return "FullLTOPostLink";
-  }
-}
+const char *to_string(ThinOrFullLTOPhase Phase);
 #endif
 
 //===----------------------------------------------------------------------===//

llvm/lib/IR/Pass.cpp

@@ -282,3 +282,21 @@ AnalysisUsage &AnalysisUsage::addRequiredTransitiveID(char &ID) {
   pushUnique(RequiredTransitive, &ID);
   return *this;
 }
+
+#ifndef NDEBUG
+const char *llvm::to_string(ThinOrFullLTOPhase Phase) {
+  switch (Phase) {
+  case ThinOrFullLTOPhase::None:
+    return "None";
+  case ThinOrFullLTOPhase::ThinLTOPreLink:
+    return "ThinLTOPreLink";
+  case ThinOrFullLTOPhase::ThinLTOPostLink:
+    return "ThinLTOPostLink";
+  case ThinOrFullLTOPhase::FullLTOPreLink:
+    return "FullLTOPreLink";
+  case ThinOrFullLTOPhase::FullLTOPostLink:
+    return "FullLTOPostLink";
+  }
+  llvm_unreachable("invalid phase");
+}
+#endif