clang-format: Improve understanding of decltype.

Before:
  SomeFunction([](decltype(x), A * a) {});

After:
  SomeFunction([](decltype(x), A *a) {});

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207961 91177308-0d34-0410-b5e6-96231b3b80d8