MS attributes do not permit empty attribute blocks.  Correctly diagnose those.
We continue to parse to ensure that we recover correctly.  Because the block is
empty, we do not need to skip any tokens.

Bonus: tweak the comment that I updated but forgot to remove the function name
in a previous commit.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@239846 91177308-0d34-0410-b5e6-96231b3b80d8