abi: add a rust-preserve-none calling convention

This is the conceptual opposite of the rust-cold calling convention and
is particularly useful in combination with the new `explicit_tail_calls`
feature.

For relatively tight loops implemented with tail calling (`become`) each
of the function with the regular calling convention is still responsible
for restoring the initial value of the preserved registers. So it is not
unusual to end up with a situation where each step in the tail call loop
is spilling and reloading registers, along the lines of:

    foo:
        push r12
        ; do things
        pop r12
        jmp next_step

This adds up quickly, especially when most of the clobberable registers
are already used to pass arguments or other uses.

I was thinking of making the name of this ABI a little less LLVM-derived
and more like a conceptual inverse of `rust-cold`, but could not come
with a great name (`rust-cold` is itself not a great name: cold in what
context? from which perspective? is it supposed to mean that the
function is rarely called?)

crates/hir-ty/src/lib.rs

@@ -230,6 +230,7 @@ pub enum FnAbi {
     Win64,
     Win64Unwind,
     X86Interrupt,
+    RustPreserveNone,
     Unknown,
 }
 
@@ -271,6 +272,7 @@ impl FnAbi {
             s if *s == sym::riscv_dash_interrupt_dash_s => FnAbi::RiscvInterruptS,
             s if *s == sym::rust_dash_call => FnAbi::RustCall,
             s if *s == sym::rust_dash_cold => FnAbi::RustCold,
+            s if *s == sym::rust_dash_preserve_dash_none => FnAbi::RustPreserveNone,
             s if *s == sym::rust_dash_intrinsic => FnAbi::RustIntrinsic,
             s if *s == sym::Rust => FnAbi::Rust,
             s if *s == sym::stdcall_dash_unwind => FnAbi::StdcallUnwind,
@@ -314,6 +316,7 @@ impl FnAbi {
             FnAbi::Rust => "Rust",
             FnAbi::RustCall => "rust-call",
             FnAbi::RustCold => "rust-cold",
+            FnAbi::RustPreserveNone => "rust-preserve-none",
             FnAbi::RustIntrinsic => "rust-intrinsic",
             FnAbi::Stdcall => "stdcall",
             FnAbi::StdcallUnwind => "stdcall-unwind",

crates/intern/src/symbol/symbols.rs

@@ -109,6 +109,7 @@ define_symbols! {
     vectorcall_dash_unwind = "vectorcall-unwind",
     win64_dash_unwind = "win64-unwind",
     x86_dash_interrupt = "x86-interrupt",
+    rust_dash_preserve_dash_none = "preserve-none",
 
     @PLAIN:
     __ra_fixup,