Simplify coro::salvageDebugInfo() (NFC-ish)

This patch removes the hand-rolled implementation of salvageDebugInfo
for cast and GEPs and replaces it with a call into
llvm::salvageDebugInfoImpl().

A side-effect of this is that additional redundant convert operations
are introduced, but those don't have any negative effect on the
resulting DWARF expression.

rdar://80227769

Differential Revision: https://reviews.llvm.org/D107384