[llvm] TableGen: Handle setting runtime libcall calling conventions (PR #144980)

[Nikita Popov via llvm-commits]

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

LGTM

> I also wanted to be able to apply a default calling conv to a RuntimeLibcallImpl, but that turned out to be annoying so leave it for later.

Having that would be pretty nice, specifying this when defining the system library is pretty awkward for the cases where the CC is specific to the libcall.

https://github.com/llvm/llvm-project/pull/144980