Minor cleanup from if to guard statement

I think the guard is a lot easier to read here, noticed while reviewing swiftlang#3171.

Author: ahoppen

Sources/SwiftParser/Lookahead.swift

@@ -266,7 +266,7 @@ extension Parser.Lookahead {
     }
 
     // If we don't have attributes, then it cannot be an accessor block.
-    if nextToken.rawTokenKind != .atSign {
+    guard self.peek(isAt: .atSign) else {
       return false
     }
 

Tests/SwiftParserTest/DeclarationTests.swift

@@ -3768,4 +3768,16 @@ final class UsingDeclarationTests: ParserTestCase {
       """
     )
   }
+
+  func testAccessorBlockAfterPatternBindingDeclWithAttribute() {
+    assertParse(
+      """
+      var x: Int = 2
+      {
+        @something
+        didSet {}
+      }
+      """
+    )
+  }
 }