Rollup merge of #150768 - bjorn3:llvm_intrinsic_no_fn_abi, r=wesleywiser

Don't compute FnAbi for LLVM intrinsics in backends

~~This removes support for `extern "unadjusted"` for anything other than LLVM intrinsics. It only makes sense in the context of calling LLVM intrinsics anyway as it exposes the way the LLVM backend internally represents types. Perhaps it should be renamed to `extern "llvm-intrinsic"`?~~

Follow up to rust-lang/rust#148533

Author: jhpratt