DebugInfo: Mangle K&R declarations for debug info linkage names

This fixes a gap in the `overloadable` attribute support (K&R declared
functions would get mangled symbol names, but that name wouldn't be
represented in the debug info linkage name field for the function) and
in -funique-internal-linkage-names (this came up in review discussion on
D98799) where K&R static declarations would not get the uniqued linkage
names.