Rollup merge of #119365 - nbdd0121:asm-goto, r=Amanieu

Add asm goto support to `asm!`

Tracking issue: #119364

This PR implements asm-goto support, using the syntax described in "future possibilities" section of [RFC2873](https://rust-lang.github.io/rfcs/2873-inline-asm.html#asm-goto).

Currently I have only implemented the `label` part, not the `fallthrough` part (i.e. fallthrough is implicit). This doesn't reduce the expressive though, since you can use label-break to get arbitrary control flow or simply set a value and rely on jump threading optimisation to get the desired control flow. I can add that later if deemed necessary.

r? ``@Amanieu``
cc ``@ojeda``

compiler/rustc_ast_lowering/src/errors.rs

@@ -261,6 +261,16 @@ pub struct InvalidAsmTemplateModifierSym {
     pub op_span: Span,
 }
 
+#[derive(Diagnostic, Clone, Copy)]
+#[diag(ast_lowering_invalid_asm_template_modifier_label)]
+pub struct InvalidAsmTemplateModifierLabel {
+    #[primary_span]
+    #[label(ast_lowering_template_modifier)]
+    pub placeholder_span: Span,
+    #[label(ast_lowering_argument)]
+    pub op_span: Span,
+}
+
 #[derive(Diagnostic, Clone, Copy)]
 #[diag(ast_lowering_register_class_only_clobber)]
 pub struct RegisterClassOnlyClobber {