Parse -> ref T as a primitive return form (#7911)

Previously `-> ref T` was parsed as a type expression whose outermost
node is a `ref` prefix operator (the same one used for `F(ref x)`
arguments), and `ReturnExprAsForm` recognized the resulting `RefTagExpr`
and turned it into a `RefForm`.

Per p5434, a return form that begins with `val`, `ref`, or `var` is not
a type expression. Parse a leading `ref` after `->` as a
`RefPrimitiveForm` node wrapping the type expression, reusing the node
produced by `form(ref T)`, and check it as a form. This prepares for
supporting `-> val T` and `-> var T` in the same way.

The only SemIR change is that the `ref_tag` instruction is no longer
emitted, and the return slot pattern refers to a `type_component_of` the
form, matching `->? form(ref T)`.

---------

Co-authored-by: Nicholas Bishop <nbishop@nbishop.net>

toolchain/parse/testdata/function/declaration.carbon

@@ -183,6 +183,20 @@ fn FormInVar(var x:? Form);
 fn SimpleReturnForm() ->? Form;
 fn ComplexReturnForm() ->? X.Y(Z);
 
+// --- ref_return.carbon
+
+fn RefReturn() -> ref u32;
+fn ComplexRefReturn() -> ref X.Y(Z);
+let a: auto = fn -> ref u32 => 0;
+
+// --- fail_ref_return_missing_type.carbon
+
+// CHECK:STDERR: fail_ref_return_missing_type.carbon:[[@LINE+4]]:22: error: expected expression [ExpectedExpr]
+// CHECK:STDERR: fn RefReturn() -> ref;
+// CHECK:STDERR:                      ^
+// CHECK:STDERR:
+fn RefReturn() -> ref;
+
 // CHECK:STDOUT: - filename: basic.carbon
 // CHECK:STDOUT:   ╭─FileStart ''
 // CHECK:STDOUT:   │ ╭─FunctionIntroducer 'fn'
@@ -505,3 +519,54 @@ fn ComplexReturnForm() ->? X.Y(Z);
 // CHECK:STDOUT:   ├─FunctionDecl ';'
 // CHECK:STDOUT:   ├─FileEnd ''
 // CHECK:STDOUT:   (root)
+// CHECK:STDOUT: - filename: ref_return.carbon
+// CHECK:STDOUT:   ╭─FileStart ''
+// CHECK:STDOUT:   │ ╭─FunctionIntroducer 'fn'
+// CHECK:STDOUT:   │ ├─IdentifierNameMaybeBeforeSignature 'RefReturn'
+// CHECK:STDOUT:   │ │ ╭─ExplicitParamListStart '('
+// CHECK:STDOUT:   │ ├─ExplicitParamList ')'
+// CHECK:STDOUT:   │ │   ╭─UnsignedIntTypeLiteral 'u32'
+// CHECK:STDOUT:   │ │ ╭─RefPrimitiveForm 'ref'
+// CHECK:STDOUT:   │ ├─ReturnType '->'
+// CHECK:STDOUT:   ├─FunctionDecl ';'
+// CHECK:STDOUT:   │ ╭─FunctionIntroducer 'fn'
+// CHECK:STDOUT:   │ ├─IdentifierNameMaybeBeforeSignature 'ComplexRefReturn'
+// CHECK:STDOUT:   │ │ ╭─ExplicitParamListStart '('
+// CHECK:STDOUT:   │ ├─ExplicitParamList ')'
+// CHECK:STDOUT:   │ │         ╭─IdentifierNameExpr 'X'
+// CHECK:STDOUT:   │ │         ├─IdentifierNameNotBeforeSignature 'Y'
+// CHECK:STDOUT:   │ │       ╭─MemberAccessExpr '.'
+// CHECK:STDOUT:   │ │     ╭─CallExprStart '('
+// CHECK:STDOUT:   │ │     ├─IdentifierNameExpr 'Z'
+// CHECK:STDOUT:   │ │   ╭─CallExpr ')'
+// CHECK:STDOUT:   │ │ ╭─RefPrimitiveForm 'ref'
+// CHECK:STDOUT:   │ ├─ReturnType '->'
+// CHECK:STDOUT:   ├─FunctionDecl ';'
+// CHECK:STDOUT:   │ ╭─LetIntroducer 'let'
+// CHECK:STDOUT:   │ │ ╭─IdentifierNameNotBeforeSignature 'a'
+// CHECK:STDOUT:   │ │ ├─BindingPatternTypeStart ':'
+// CHECK:STDOUT:   │ │ ├─AutoTypeLiteral 'auto'
+// CHECK:STDOUT:   │ ├─LetBindingPattern ':'
+// CHECK:STDOUT:   │ ├─LetInitializer '='
+// CHECK:STDOUT:   │ │   ╭─LambdaIntroducer 'fn'
+// CHECK:STDOUT:   │ │   │   ╭─UnsignedIntTypeLiteral 'u32'
+// CHECK:STDOUT:   │ │   │ ╭─PrefixOperatorRef 'ref'
+// CHECK:STDOUT:   │ │   ├─ReturnType '->'
+// CHECK:STDOUT:   │ │ ╭─LambdaDefinitionStart '=>'
+// CHECK:STDOUT:   │ │ ├─IntLiteral '0'
+// CHECK:STDOUT:   │ ├─Lambda 'fn'
+// CHECK:STDOUT:   ├─LetDecl ';'
+// CHECK:STDOUT:   ├─FileEnd ''
+// CHECK:STDOUT:   (root)
+// CHECK:STDOUT: - filename: fail_ref_return_missing_type.carbon
+// CHECK:STDOUT:   ╭─FileStart ''
+// CHECK:STDOUT:   │ ╭─FunctionIntroducer 'fn'
+// CHECK:STDOUT:   │ ├─IdentifierNameMaybeBeforeSignature 'RefReturn'
+// CHECK:STDOUT:   │ │ ╭─ExplicitParamListStart '('
+// CHECK:STDOUT:   │ ├─ExplicitParamList ')'
+// CHECK:STDOUT:   │ │   ╭─InvalidParse ';' has_error
+// CHECK:STDOUT:   │ │ ╭─RefPrimitiveForm 'ref' has_error
+// CHECK:STDOUT:   │ ├─ReturnType '->'
+// CHECK:STDOUT:   ├─FunctionDecl ';'
+// CHECK:STDOUT:   ├─FileEnd ''
+// CHECK:STDOUT:   (root)