[PATCH] Linker: Replace overridden subprograms

[Duncan P. N. Exon Smith]

When a function gets replaced by `ModuleLinker`, ensure that the "first"
subprogram pointing to it is updated to be the *same* subprogram pointed
to by the `!dbg` references in that function.  That means it should come
from the "first" subprogram in a linked-in compile unit.

This is a stop-gap "fix" for PR21910.  Ideally we wouldn't have two
different subprograms around, but it's not easy to fix that.  In the
meantime, this fixes bootstraps that hit assertions in
`LexicalScopes::initialize()` due to the explicit instantiations in
`lib/IR/Dominators.cpp` eventually getting replaced by -argpromotion.

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