[clang] [clang] Allow delayed function instantiation at TU end if initial instantiation fails (PR #117167)

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StefanPaulet wrote:

I did not remove the `AtEndOfTU` parameter from `Sema::InstantiateVariableDefinition` as it breaks some tests related to diagnostics (e.g. `SemaCXX/undefined-internal.cpp`) , as previously the parameter could have been set to `true` although the end of the translation unit was not yet reached.

https://github.com/llvm/llvm-project/pull/117167