[6.0 cherry-pick][embedded] Don't retain subclass vtable entries if the base method was DCE'd

[kubamracek]

• Explanation: Under -O/-Osize, subclasses defined across modules in Embedded Swift can result in missing symbols at link time, when an unused base class method is eliminated by dead function elimination. To fix this, we allow dead function elimination to correctly eliminate the method in the subclass too. [embedded] Don't retain subclass vtable entries if the base method was DCE'd #72898
• Scope: Embedded Swift only.
• Risk: Low, all changes only apply to Embedded Swift.
• Testing: Added test case.
• Issue: rdar://126044717, Undefined reference when subclassing class with internal/private methods #72890
• Reviewer: @eeckstein on [embedded] Don't retain subclass vtable entries if the base method was DCE'd #72898

[kubamracek]

@swift-ci please test

[kubamracek]

@swift-ci please test