[clang-format] Fix build on gcc < 7 introduced in rG9363aa9.

This fixes a bogus build error on gcc, e.g. https://lab.llvm.org/buildbot/#/builders/110/builds/2973.

/home/ssglocal/clang-cmake-x86_64-avx2-linux/clang-cmake-x86_64-avx2-linux/llvm/clang/lib/Format/TokenAnnotator.cpp:3097:53: error: binding ‘const clang::SourceRange’ to reference of type ‘clang::SourceRange&’ discards qualifiers
   auto HasExistingWhitespace = [&Whitespace = Right.WhitespaceRange]() {
                                                     ^