Improve asm support

Including:

 - Infer `label {}` and `const` operands.
 - Correctly handle unsafe check inside `label {}`.
 - Fix an embarrassing parser typo that cause labels to never be part of the AST

crates/parser/src/grammar/expressions/atom.rs

@@ -381,10 +381,14 @@ fn parse_asm_expr(p: &mut Parser<'_>, m: Marker) -> Option<CompletedMarker> {
             op.complete(p, ASM_REG_OPERAND);
             op_n.complete(p, ASM_OPERAND_NAMED);
         } else if p.eat_contextual_kw(T![label]) {
+            // test asm_label
+            // fn foo() {
+            //     builtin#asm("", label {});
+            // }
             dir_spec.abandon(p);
             block_expr(p);
-            op.complete(p, ASM_OPERAND_NAMED);
-            op_n.complete(p, ASM_LABEL);
+            op.complete(p, ASM_LABEL);
+            op_n.complete(p, ASM_OPERAND_NAMED);
         } else if p.eat(T![const]) {
             dir_spec.abandon(p);
             expr(p);

crates/parser/test_data/generated/runner.rs

@@ -21,6 +21,8 @@ mod ok {
     #[test]
     fn asm_expr() { run_and_expect_no_errors("test_data/parser/inline/ok/asm_expr.rs"); }
     #[test]
+    fn asm_label() { run_and_expect_no_errors("test_data/parser/inline/ok/asm_label.rs"); }
+    #[test]
     fn assoc_const_eq() {
         run_and_expect_no_errors("test_data/parser/inline/ok/assoc_const_eq.rs");
     }

crates/parser/test_data/parser/inline/ok/asm_label.rast

@@ -0,0 +1,37 @@
+SOURCE_FILE
+  FN
+    FN_KW "fn"
+    WHITESPACE " "
+    NAME
+      IDENT "foo"
+    PARAM_LIST
+      L_PAREN "("
+      R_PAREN ")"
+    WHITESPACE " "
+    BLOCK_EXPR
+      STMT_LIST
+        L_CURLY "{"
+        WHITESPACE "\n    "
+        EXPR_STMT
+          ASM_EXPR
+            BUILTIN_KW "builtin"
+            POUND "#"
+            ASM_KW "asm"
+            L_PAREN "("
+            LITERAL
+              STRING "\"\""
+            COMMA ","
+            WHITESPACE " "
+            ASM_OPERAND_NAMED
+              ASM_LABEL
+                LABEL_KW "label"
+                WHITESPACE " "
+                BLOCK_EXPR
+                  STMT_LIST
+                    L_CURLY "{"
+                    R_CURLY "}"
+            R_PAREN ")"
+          SEMICOLON ";"
+        WHITESPACE "\n"
+        R_CURLY "}"
+  WHITESPACE "\n"

crates/parser/test_data/parser/inline/ok/asm_label.rs

@@ -0,0 +1,3 @@
+fn foo() {
+    builtin#asm("", label {});
+}