clang-format: Fix pointer/reference detection after decltype.

Before:
  [](const decltype(*a) & value) {}

After:
  [](const decltype(*a)& value) {}

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210643 91177308-0d34-0410-b5e6-96231b3b80d8