swift234

SILGen: Use the base SILDeclRef for super_method override constant info

getOverriddenVTableEntry only goes one level up in the class hierarchy,
but getConstantOverrideInfo requires that the next level up not itself be an
override.

A little bit of refactoring:

SILDeclRef::getOverriddenVTableEntry()
  -> SILDeclRef::getNextOverriddenVTableEntry()

static findOverriddenFunction()
  -> SILDeclRef::getBaseOverriddenVTableEntry()

rdar://problem/22749732