[clang] Reland [Clang] skip default argument instantiation for non-defining friend declarations to meet [dcl.fct.default] p4 (PR #115487)

[Younan Zhang via cfe-commits]

https://github.com/zyn0217 approved this pull request.

LGTM modulo 1 nit.
@mizvekov are you happy with it?

https://github.com/llvm/llvm-project/pull/115487