[clang-format] Add support for C++17 structured bindings.

Summary:
Before:
```
    auto[a, b] = f();
```

After:
```
    auto [a, b] = f();
```
or, if SpacesInSquareBrackets is true:
```
    auto [ a, b ] = f();
```

Reviewers: djasper

Reviewed By: djasper

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D37132

llvm-svn: 312723