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c05126bdfc
Summary: The LTO pass and LLD linker have logic in them that forces extraction and prevent internalization of needed runtime calls. However, these currently take all RTLibcalls into account, even if the target does not support them. The target opts-out of a libcall if it sets its name to nullptr. This patch pulls this logic out into a class in the header so that LTO / lld can use it to determine if a symbol actually needs to be kept. This is important for targets like AMDGPU that want to be able to use `lld` to perform the final link step, but does not want the overhead of uncalled functions. (This adds like a second to the link time trivially)