[nnc] Use given kernel function name while emitting code (#67781)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67781
Update `LLVMCodeGen` in NNC to use the given kernel function name while emitting code.
This was earlier committed as D31445799 (https://github.com/pytorch/pytorch/commit/c30dc52739c083d8b10f36ed3e84f765f349fe9b) and got reverted as part of a stack of diffs that included a cache for `PyTorchLLVMJIT`, which was the likely culprit.
Test Plan:
```
buck test mode/opt //caffe2/test/cpp/tensorexpr:tensorexpr -- --exact 'caffe2/test/cpp/tensorexpr:tensorexpr - LLVM.CodeGenKernelFuncName'
```
Reviewed By: ZolotukhinM, bdhirsh
Differential Revision: D32145958
fbshipit-source-id: 5f4e0400c4fa7cabce5b91e6de2a294fa0cad88e
