Add support for emitting functions with coldcc in LLVM

The eventual goal is to try using this for things like the internal panicking stuff, to see whether it helps.

compiler/rustc_codegen_llvm/src/abi.rs

@@ -393,6 +393,7 @@ impl<'ll, 'tcx> FnAbiLlvmExt<'ll, 'tcx> for FnAbi<'tcx, Ty<'tcx>> {
     fn llvm_cconv(&self) -> llvm::CallConv {
         match self.conv {
             Conv::C | Conv::Rust | Conv::CCmseNonSecureCall => llvm::CCallConv,
+            Conv::RustCold => llvm::ColdCallConv,
             Conv::AmdGpuKernel => llvm::AmdGpuKernel,
             Conv::AvrInterrupt => llvm::AvrInterrupt,
             Conv::AvrNonBlockingInterrupt => llvm::AvrNonBlockingInterrupt,