[DebugInfo] Make DISubprogram's hashing always produce the same result (#90770)

A DISubprogram's hashing algorithm takes into account its Scope. A Scope
can be a temporary though which can be replaced later on during
compilation. This means that the hashing algorithm for a DISubprogram
could produce a different hash before/after the Scope has changed. Fix
this by checking the Scope's linkage name instead, which should always
be the same.

rdar://127004707