clang-format: Prefer wrapping a lambda's body over the lambda's return type.

Before:
  aaaaaaaaaaaaaaaaaaaaaa(
      [](aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa &aaa)
          -> aaaaaaaaaaaaaaaaaaaaa { return aaaaaaaaaaaaaaaaa; });

After:
  aaaaaaaaaaaaaaaaaaaaaa(
      [](aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa &aaa) -> aaaaaaaaaaaaaaaaaaaaa {
        return aaaaaaaaaaaaaaaaa;
      });

llvm-svn: 230942