Replace 'unsigned short' with 'uint16_t' in a packed data structure for consistency with other fields and to be explicit about bit count.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186796 91177308-0d34-0410-b5e6-96231b3b80d8
Const-correct some iterators. No functional change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186797 91177308-0d34-0410-b5e6-96231b3b80d8