Allows customization of builtin functions under FatLTO #119348
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Fixes #118609.
Currently, we are using LTO in a way that is similar to LLVM's legacy LTO. This approach does not allow the caller to decide which symbol to use. LLVM's new LTO puts symbol resolution in the hands of the caller.
This PR uses a similar approach to the new LTO to select the appropriate builtin symbol. I'm not dealing with the weak symbol in the inline asm. But this can also be done with
.lto_discard
. I'm not sure it's worth implementing it.r? @bjorn3