Improve clang-format's memoization behavior.

Deeply nested expressions basically break clang-format's memoization.
This patch slightly improves the situations and makes expressions like

  aaaaa(aaaaa(aaaaa(aaaaa(aaaaa(aaaaa(aaaaa(aaaaa(
      aaaaa(aaaaa(aaaaa(aaaaa(aaaaa(aaaaa(aaaaa(aaaaa(aaaaa(aaaaa(aaaaa(
          aaaaa(aaaaa(aaaaa(aaaaa(aaaaa(aaaaa(aaaaa(aaaaa(aaaaa(aaaaa(
              aaaaa(aaaaa(aaaaa(aaaaa(aaaaa(aaaaa(aaaaa(aaaaa(aaaaa(
                  aaaaa(aaaaa())))))))))))))))))))))))))))))))))))))));

work.

llvm-svn: 180264