clang-format: Support chained dereferenced assignments.

Before:
  x = * a(x) = *a(y);

After:
  x = *a(x) = *a(y);

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215632 91177308-0d34-0410-b5e6-96231b3b80d8