[Flang] funderscoring intermittent failure fix

There is an intermittent failure in the tests for the funderscoring driver option reported in (https://lab.llvm.org/buildbot/#/builders/21/builds/78228) that is caused by an uninitialized member variable.

Reviewed By: kkwli0

Differential Revision: https://reviews.llvm.org/D158187

flang/lib/Optimizer/Transforms/ExternalNameConversion.cpp

@@ -165,7 +165,7 @@ class ExternalNameConversionPass
 
 private:
   bool appendUnderscores;
-  bool usePassOpt;
+  bool usePassOpt = false;
 };
 } // namespace