Fix opt.exe external lib loading (#5049)

tools/clang/tools/dxopt/dxopt.cpp

@@ -316,7 +316,7 @@ int main(int argc, const char **argv) {
 
     if (externalLib) {
       CW2A externalFnA(externalFn, CP_UTF8);
-      CW2A externalLibA(externalFn, CP_UTF8);
+      CW2A externalLibA(externalLib, CP_UTF8);
       IFT(g_DxcSupport.InitializeForDll(externalLibA, externalFnA));
     }
     else {