[MLIR] Attempt to fix sanitizer issue (access of uninitialized member)

Follow-up to llvm#156825 ; a member wasn't always initialized, it's not clear to me
why this is needed though, so this fix may not be the right one, we'll monitor
the bots.

Author: joker-eph

mlir/include/mlir/Tools/mlir-opt/MlirOptMain.h

@@ -264,7 +264,7 @@ class MlirOptMainConfig {
   bool allowUnregisteredDialectsFlag = false;
 
   /// Remark format
-  RemarkFormat remarkFormatFlag;
+  RemarkFormat remarkFormatFlag = REMARK_FORMAT_STDOUT;
   /// Remark file to output to
   std::string remarksOutputFileFlag = "";
   /// Remark filters