[clang] [llvm] [debuginfo][coro] Fix linkage name for clones of coro functions (PR #141889)

[Adrian Vogelsgesang via cfe-commits]

vogelsgesang wrote:

> Since the change is almost about swift, CC @adrian-prantl

Note that this change should be a NFC for Swift. Changing the `DISubprogram::get` call to `Decl->clone() + replaceLinkageName` just simplifies the code, but does not change its semantics.

This PR primarily enables the same behavior which already existed in Swift also for C++.

https://github.com/llvm/llvm-project/pull/141889