Don't use clang's callback to hack the cancellation branch

This conflicted with my hack doing the same thing: leading to a
use-after-free of the old terminator. I think it is more helpful to do
this in OpenMPIRBuilder than replicating the hack in callbacks in both
clang and mlir.

Author: tblah

clang/lib/CodeGen/CGStmtOpenMP.cpp

@@ -4345,8 +4345,9 @@ void CodeGenFunction::EmitOMPSectionsDirective(const OMPSectionsDirective &S) {
     using InsertPointTy = llvm::OpenMPIRBuilder::InsertPointTy;
     using BodyGenCallbackTy = llvm::OpenMPIRBuilder::StorableBodyGenCallbackTy;
 
-    auto FiniCB = [this](InsertPointTy IP) {
-      OMPBuilderCBHelpers::FinalizeOMPRegion(*this, IP);
+    auto FiniCB = [](InsertPointTy IP) {
+      // Don't FinalizeOMPRegion because this is done inside of OMPIRBuilder for
+      // sections.
       return llvm::Error::success();
     };