MS ABI: Correct layout for empty records

Empty records do not always have size equivalent to their alignment.
They only do so when their alignment is at least as large as the minimum
empty struct size: 1 byte in C++ and 4 bytes in C.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@218661 91177308-0d34-0410-b5e6-96231b3b80d8