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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@180264 91177308-0d34-0410-b5e6-96231b3b80d8