Update the linkage name of coro-split functions in the debug info.

This patch updates the linkage name in the DISubprogram of coro-split
functions, which is particularly important for Swift, where the
funclets have a special name mangling. This patch does not affect C++
coroutines, since the DW_AT_specification is expected to hold the
(original) linkage name. I believe this is mostly due to limitations
in AsmPrinter, so we might be able to relax this restriction in the
future.

Differential Revision: https://reviews.llvm.org/D99693