Silence a warning from MSVC "14" by making an enum unsigned

It says there is a narrowing conversion when we assign it to an unsigned
3 bit bitfield.

Also, use unsigned instead of size_t for the Size field of the struct in
question. Otherwise they won't run together in MSVC or clang-cl.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@221019 91177308-0d34-0410-b5e6-96231b3b80d8