[llvm] [ELFYAML] Introduce the external interface `CustomSection` (PR #115707)

[NAKAMURA Takumi via llvm-commits]

chapuni wrote:

@ornata Imagine that `clangParse` would depend on `X86CodeGen` just for parsing MSC style asm. I think we have to avoid adhoc deps as possible and keep clean **as the library**.

https://github.com/llvm/llvm-project/pull/115707