Improve clang-format's understanding of casts.

This fixes llvm.org/PR14684.

Before: int *pa = (int *) & a;
After:  int *pa = (int *)&a;

We still don't understand all kinds of casts. I added a FIXME to
address that.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172056 91177308-0d34-0410-b5e6-96231b3b80d8