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

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cor3ntin wrote:

The behavior I would expect is

`callable` is instantiated. It has a trailing return type so only the declaration of `foo` is instantiated, and that also has a return type so we do not need the definition. `foo` is marked referenced / odr-used. That should be enough to  force an instantiation of the definition at the end of the TU. 

Can you try to figure out what would cause an eager instantiation of `foo`'s definition?

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