[llvm] Port Swift's merge function pass to llvm: merging functions that differ in constants (PR #68235)

[Nikita Popov via llvm-commits]

nikic wrote:

You likely need to start a new one, I don't think GitHub allows reopening merged PRs.

A couple obvious issues from scrolling through the code:
 * Typed pointer support sprinkled about, including use of deprecated APIs. Needs to be removed. Also applies to tests.
 * Incomplete pointer authentication support sprinkled about. Unless there is full upstream support for this, it needs to be removed from this patch completely.
 * Drop `llvm::` prefixes. Not needed/used in LLVM.

TBH this PR doesn't really look in a reviewable state yet. 

https://github.com/llvm/llvm-project/pull/68235