Fix one memory leak in the MLIRParser by using std::unique_ptr to hold the new block pointer

[mehdi_amini]

Summary

This is NFC when there is no parsing error.

mlir/lib/Parser/Parser.cpp

ParseResult OperationParser::parseRegion(
   if (entryArguments.empty() && consumeIf(Token::r_brace))
     return success();
   auto currentPt = opBuilder.saveInsertionPoint();
 
   // Push a new named value scope.
   pushSSANameScope(isIsolatedNameScope);
 
   // Parse the first block directly to allow for it to be unnamed.
-  Block *block = new Block();
+  auto owning_block = std::make_unique<Block>();
+  Block *block = owning_block.get();
 
   // Add arguments to the entry block.
   if (!entryArguments.empty()) {
     for (auto &placeholderArgPair : entryArguments) {
       auto &argInfo = placeholderArgPair.first;
       // Ensure that the argument was not already defined.
       if (auto defLoc = getReferenceLoc(argInfo.name, argInfo.number)) {
         return emitError(argInfo.loc, "region entry argument '" + argInfo.name +
                                           "' is already in use")
                    .attachNote(getEncodedSourceLocation(*defLoc))
                << "previously referenced here";
       }
       if (addDefinition(placeholderArgPair.first,
                         block->addArgument(placeholderArgPair.second))) {
-        delete block;
         return failure();
       }
     }
 
     // If we had named arguments, then don't allow a block name.
     if (getToken().is(Token::caret_identifier))
       return emitError("invalid block name in region with named arguments");
   }
 
   if (parseBlock(block)) {
-    delete block;
     return failure();
   }
 
   // Verify that no other arguments were parsed.
   if (!entryArguments.empty() &&
       block->getNumArguments() > entryArguments.size()) {
-    delete block;
     return emitError("entry block arguments were already defined");
   }
 
   // Parse the rest of the region.
-  region.push_back(block);
+  region.push_back(owning_block.release());
   if (parseRegionBody(region))
     return failure();
 
   // Pop the SSA value scope for this region.
   if (popSSANameScope())
     return failure();
 
   // Reset the original insertion point.