clang-format: Understand parameter pack initialization.

Before:
  Constructor(A... a) : a_(X<A> { std::forward<A>(a) }...) {}

After:
  Constructor(A... a) : a_(X<A>{std::forward<A>(a)}...) {}

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214720 91177308-0d34-0410-b5e6-96231b3b80d8