Remove parsing of "value-like" macro declarations

This `macro m: T` macro syntax was present in early macro proposals,
but has not been used in a while and isn't in the accepted form. Remove
it.

Author: DougGregor

Sources/SwiftParser/Declarations.swift

@@ -2164,15 +2164,7 @@ extension Parser {
     }
 
     // Macro signature, which is either value-like or function-like.
-    let signature: RawMacroDeclSyntax.Signature
-    if let colon = self.consume(if: .colon) {
-      let type = self.parseType()
-      signature = .valueLike(
-        RawTypeAnnotationSyntax(colon: colon, type: type, arena: self.arena)
-      )
-    } else {
-      signature = .functionLike(self.parseFunctionSignature())
-    }
+    let signature = self.parseFunctionSignature()
 
     // Initializer, if any.
     let definition: RawInitializerClauseSyntax?

Sources/SwiftSyntaxMacros/MacroReplacement.swift

@@ -114,11 +114,7 @@ fileprivate class ParameterReplacementVisitor: SyntaxAnyVisitor {
   // of a macro.
   override func visit(_ node: IdentifierExprSyntax) -> SyntaxVisitorContinueKind {
     let identifier = node.identifier
-
-    // FIXME: This will go away.
-    guard case let .functionLike(signature) = macro.signature else {
-      return .visitChildren
-    }
+    let signature = macro.signature
 
     let matchedParameter = signature.input.parameterList.enumerated().first { (index, parameter) in
       if identifier.text == "_" {

Tests/SwiftParserTest/DeclarationTests.swift

@@ -1334,12 +1334,18 @@ final class DeclarationTests: XCTestCase {
   func testMacroDecl() {
     AssertParse(
       """
-      macro m1: Int = A.M1
+      macro m11️⃣: Int = A.M1
       macro m2(_: Int) = A.M2
       macro m3(a b: Int) -> Int = A.M3
-      macro m4<T>: T = A.M4 where T.Assoc: P
+      macro m4<T>2️⃣: T = A.M4 where T.Assoc: P
       macro m5<T: P>(_: T)
-      """
+      """,
+      diagnostics: [
+        DiagnosticSpec(locationMarker: "1️⃣", message: "expected parameter clause in function signature"),
+        DiagnosticSpec(locationMarker: "1️⃣", message: "unexpected code ': Int = A.M1' before macro"),
+        DiagnosticSpec(locationMarker: "2️⃣", message: "expected parameter clause in function signature"),
+        DiagnosticSpec(locationMarker: "2️⃣", message: "unexpected code ': T = A.M4 where T.Assoc: P' before macro")
+      ]
     )
 
     AssertParse(