[mlir] fix memory leak on failure path in parser

[ftynse]

Summary

Forward references to blocks lead to Blocks being allocated in the
parser, but they are not necessarily included into a region if parsing
fails, leading to a leak. Clean them up in parser destructor.

mlir/lib/Parser/Parser.cpp

 
 OperationParser::~OperationParser() {
   for (auto &fwd : forwardRefPlaceholders) {
     // Drop all uses of undefined forward declared reference and destroy
     // defining operation.
     fwd.first.dropAllUses();
     fwd.first.getDefiningOp()->destroy();
   }
+  for (const auto &scope : forwardRef) {
+    for (const auto &fwd : scope) {
+      // Delete all blocks that were created as forward references but never
+      // included into a region.
+      fwd.first->dropAllUses();
+      delete fwd.first;
+    }
+  }
 }
 
 /// After parsing is finished, this function must be called to see if there are
 /// any remaining issues.
 ParseResult OperationParser::finalize() {
   // Check for any forward references that are left.  If we find any, error
   // out.
   if (!forwardRefPlaceholders.empty()) {