[clang-format] Adjust space around &/&& of structured bindings

Keep space before or after the &/&& tokens, but not both. For example,
  auto [x,y] = a;
  auto &[xr, yr] = a; // LLVM style
  auto& [xr, yr] = a; // google style

Differential Revision:https://reviews.llvm.org/D35743

llvm-svn: 314264