Fix "not all control paths return a value" warning on MSVC builds.

Use llvm_unreachable as typeName(Metric::MetricType T) should handle all enum values.

clang-tools-extra/clangd/support/Trace.cpp

@@ -225,6 +225,7 @@ private:
     case Metric::Distribution:
       return "d";
     }
+    llvm_unreachable("Unknown Metric::MetricType enum");
   }
 
   static bool needsQuote(llvm::StringRef Text) {