Auto merge of #115260 - scottmcm:not-quite-so-cold, r=WaffleLapkin

Use `preserve_mostcc` for `extern "rust-cold"`

As experimentation in #115242 has shown looks better than `coldcc`.  Notably, clang exposes `preserve_most` (https://clang.llvm.org/docs/AttributeReference.html#preserve-most) but not `cold`, so this change should put us on a better-supported path.

And *don't* use a different convention for cold on Windows, because that actually ends up making things worse. (See comment in the code.)

cc tracking issue #97544

compiler/rustc_codegen_llvm/src/abi.rs

@@ -571,7 +571,9 @@ impl From<Conv> for llvm::CallConv {
             Conv::C | Conv::Rust | Conv::CCmseNonSecureCall | Conv::RiscvInterrupt { .. } => {
                 llvm::CCallConv
             }
-            Conv::RustCold => llvm::ColdCallConv,
+            Conv::Cold => llvm::ColdCallConv,
+            Conv::PreserveMost => llvm::PreserveMost,
+            Conv::PreserveAll => llvm::PreserveAll,
             Conv::AmdGpuKernel => llvm::AmdGpuKernel,
             Conv::AvrInterrupt => llvm::AvrInterrupt,
             Conv::AvrNonBlockingInterrupt => llvm::AvrNonBlockingInterrupt,

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -83,12 +83,17 @@ pub enum LLVMModFlagBehavior {
 // Consts for the LLVM CallConv type, pre-cast to usize.
 
 /// LLVM CallingConv::ID. Should we wrap this?
+///
+/// See <https://github.com/llvm/llvm-project/blob/main/llvm/include/llvm/IR/CallingConv.h>
 #[derive(Copy, Clone, PartialEq, Debug)]
 #[repr(C)]
 pub enum CallConv {
     CCallConv = 0,
     FastCallConv = 8,
     ColdCallConv = 9,
+    PreserveMost = 14,
+    PreserveAll = 15,
+    Tail = 18,
     X86StdcallCallConv = 64,
     X86FastcallCallConv = 65,
     ArmAapcsCallConv = 67,