[clang] fixing -ast-print for variadic parameter pack in lambda capture

Summary:
currently for:
```
 template<typename ... T>
void f(T... t) {
  auto l = [t...]{};
}
```
`clang -ast-print file.cpp`
outputs:

```
template <typename ...T> void f(T ...t) {
    auto l = [t]         {
        }
;
}
```
notice that there is not `...` in the capture list of the lambda. this patch fixes this issue. and add test for it.

Patch by Tyker

Reviewers: rsmith

Reviewed By: rsmith

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61556

llvm-svn: 359980

clang/lib/AST/StmtPrinter.cpp

@@ -1895,6 +1895,9 @@ void StmtPrinter::VisitLambdaExpr(LambdaExpr *Node) {
       llvm_unreachable("VLA type in explicit captures.");
     }
 
+    if (C->isPackExpansion())
+      OS << "...";
+
     if (Node->isInitCapture(C))
       PrintExpr(C->getCapturedVar()->getInit());
   }

clang/test/AST/ast-printer-lambda.cpp

@@ -0,0 +1,36 @@
+// RUN: %clang_cc1 -ast-print -std=c++17 %s | FileCheck %s
+
+struct S {
+template<typename ... T>
+void test1(int i, T... t) {
+{
+  auto lambda = [i]{};
+  //CHECK: [i] {
+}
+{
+  auto lambda = [=]{};
+  //CHECK: [=] {
+}
+{
+  auto lambda = [&]{};
+  //CHECK: [&] {
+}
+{
+  auto lambda = [t..., i]{};
+  //CHECK: [t..., i] {
+}
+{
+  auto lambda = [&t...]{};
+  //CHECK: [&t...] {
+}
+{
+  auto lambda = [this, &t...]{};
+  //CHECK: [this, &t...] {
+}
+{
+  auto lambda = [t..., this]{};
+  //CHECK: [t..., this] {
+}
+}
+
+};