[clang] [CIR] Implement NYI: function declaration that forces code gen (PR #194663)

[Zaky Hermawan via cfe-commits]

ZakyHermawan wrote:

Based on @andykaylor's suggestion, I am converting this PR to a draft. It will be reopened once the necessary changes for `setGlobalVisibility` have been merged.

https://github.com/llvm/llvm-project/pull/194663