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?)

compiler/rustc_public/src/abi.rs

@@ -432,6 +432,7 @@ pub enum CallConvention {
     Cold,
     PreserveMost,
     PreserveAll,
+    PreserveNone,
 
     Custom,
 

compiler/rustc_public/src/ty.rs

@@ -1139,6 +1139,7 @@ pub enum Abi {
     RustCold,
     RiscvInterruptM,
     RiscvInterruptS,
+    RustPreserveNone,
     RustInvalid,
     Custom,
 }

compiler/rustc_public/src/unstable/convert/internal.rs

@@ -615,6 +615,7 @@ impl RustcInternal for Abi {
             Abi::RustInvalid => rustc_abi::ExternAbi::RustInvalid,
             Abi::RiscvInterruptM => rustc_abi::ExternAbi::RiscvInterruptM,
             Abi::RiscvInterruptS => rustc_abi::ExternAbi::RiscvInterruptS,
+            Abi::RustPreserveNone => rustc_abi::ExternAbi::RustPreserveNone,
             Abi::Custom => rustc_abi::ExternAbi::Custom,
         }
     }

compiler/rustc_public/src/unstable/convert/stable/abi.rs

@@ -123,6 +123,7 @@ impl<'tcx> Stable<'tcx> for CanonAbi {
             CanonAbi::C => CallConvention::C,
             CanonAbi::Rust => CallConvention::Rust,
             CanonAbi::RustCold => CallConvention::Cold,
+            CanonAbi::RustPreserveNone => CallConvention::PreserveNone,
             CanonAbi::Custom => CallConvention::Custom,
             CanonAbi::Arm(arm_call) => match arm_call {
                 ArmCall::Aapcs => CallConvention::ArmAapcs,

compiler/rustc_public/src/unstable/convert/stable/ty.rs

@@ -1020,6 +1020,7 @@ impl<'tcx> Stable<'tcx> for rustc_abi::ExternAbi {
             ExternAbi::RustCall => Abi::RustCall,
             ExternAbi::Unadjusted => Abi::Unadjusted,
             ExternAbi::RustCold => Abi::RustCold,
+            ExternAbi::RustPreserveNone => Abi::RustPreserveNone,
             ExternAbi::RustInvalid => Abi::RustInvalid,
             ExternAbi::RiscvInterruptM => Abi::RiscvInterruptM,
             ExternAbi::RiscvInterruptS => Abi::RiscvInterruptS,