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

[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@359980 91177308-0d34-0410-b5e6-96231b3b80d8