[llvm] Add support for ref.func to AsmParser/MC (PR #163326)

[Demetrius Kanios via llvm-commits]

QuantumSegfault wrote:

It actually is related to the use case I described.

Long story short, I'm passing several `funcref` to the embedder, which it will call it at later time (these only passed out, never back in; stored and reused indefinitely). Using `ref.func` directly replace unnecessary `i32.const` + `table.get` from `__indirect_function_table` (which is a little awkward in LLVM IR).

https://github.com/llvm/llvm-project/pull/163326