Adjust to swift-syntax parsing freestanding macros as declarations more often

Author: DougGregor

lib/ASTGen/Sources/ASTGen/Macros.swift

@@ -169,10 +169,16 @@ func evaluateMacro(
     switch macroPtr.pointee.macro {
     // Handle expression macro.
     case let exprMacro as ExpressionMacro.Type:
-      guard let parentExpansion = parentSyntax.as(MacroExpansionExprSyntax.self) else {
-        print("not on a macro expansion node: \(token.recursiveDescription)")
+      let parentExpansion: MacroExpansionExprSyntax
+      if let expansionExpr = parentSyntax.as(MacroExpansionExprSyntax.self) {
+        parentExpansion = expansionExpr
+      } else if let expansionDecl = parentSyntax.as(MacroExpansionDeclSyntax.self) {
+        parentExpansion = expansionDecl.asMacroExpansionExpr()
+      } else {
+        print("not on a macro expansion node: \(parentSyntax.recursiveDescription)")
         return -1
       }
+
       macroName = parentExpansion.macro.withoutTrivia().description
       evaluatedSyntax = Syntax(try exprMacro.expansion(of: parentExpansion, in: &context))