clang-format: Preserve meaning of trailing comments on parameters.

Formatting:
  SomeFunction(a,
            b, // comment
            c);

Before:
  SomeFunction(a, b, // comment
               c);

After:
  SomeFunction(a,
               b, // comment
               c);

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204456 91177308-0d34-0410-b5e6-96231b3b80d8