Rollup merge of #147259 - Zalathar:node-in-context, r=nnethercote

cg_llvm: Use helper methods for all calls to `LLVMMDNodeInContext2`

Originally I was only planning on extracting an `md_node_in_context` method, but then I noticed that all callers of `LLVMMDNodeInContext2` could be covered by a small number of additional helper methods.

There should be no change in compiler output.

Author: Zalathar