[Macros] Add a test of a variadic "print" macro.

Macros can use variadic generics, and don't need runtime support for
variadic generics to do so. Add an example "print".

Author: DougGregor

test/Macros/Inputs/variadic_macros.swift

@@ -0,0 +1,22 @@
+import SwiftDiagnostics
+import SwiftOperators
+import SwiftSyntax
+import SwiftSyntaxBuilder
+import _SwiftSyntaxMacros
+
+public struct PrintMacro: ExpressionMacro {
+  public static func expansion(
+    of node: MacroExpansionExprSyntax, in context: inout MacroExpansionContext
+  ) -> ExprSyntax {
+    let printCalls = node.argumentList.map {
+      "print(\($0.expression))"
+    }.joined(separator: "\n")
+    return
+      """
+      {
+      \(raw: printCalls)
+      }()
+      """
+  }
+}
+

test/Macros/macro_expand_variadic.swift

@@ -0,0 +1,23 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift -I %swift-host-lib-dir -L %swift-host-lib-dir -emit-library -o %t/%target-library-name(MacroDefinition) -module-name=MacroDefinition %S/Inputs/variadic_macros.swift -g -no-toolchain-stdlib-rpath
+// RUNx: %target-swift-frontend -dump-ast -enable-experimental-feature Macros -enable-experimental-feature VariadicGenerics -load-plugin-library %t/%target-library-name(MacroDefinition) -I %swift-host-lib-dir %s -module-name MacroUser 2>&1 | %FileCheck --check-prefix CHECK-AST %s
+// RUN: %target-typecheck-verify-swift -enable-experimental-feature Macros -enable-experimental-feature VariadicGenerics -load-plugin-library %t/%target-library-name(MacroDefinition) -I %swift-host-lib-dir -module-name MacroUser -DTEST_DIAGNOSTICS -swift-version 5
+// RUN: %target-build-swift -enable-experimental-feature Macros -enable-experimental-feature VariadicGenerics -load-plugin-library %t/%target-library-name(MacroDefinition) -I %swift-host-lib-dir -L %swift-host-lib-dir %s -o %t/main -module-name MacroUser -swift-version 5
+// RUN: %target-run %t/main | %FileCheck %s
+// REQUIRES: executable_test
+
+// FIXME: Swift parser is not enabled on Linux CI yet.
+// REQUIRES: OS=macosx
+
+@expression macro varPrint<Value...>(_ value: Value...) = MacroDefinition.PrintMacro
+
+func testIt() {
+  // CHECK: hello
+  // CHECK: [1, 2, 3, 4, 5]
+  #varPrint("hello", [1, 2, 3, 4, 5])
+
+  // CHECK: world
+  #varPrint("world")
+}
+
+testIt()