Fix representation of __attribute__((nonnull)) to support correctly modeling

Fix representation of __attribute__((nonnull)) to support correctly modeling
the no-arguments case. Don't expand this to an __attribute__((nonnull(A, B,
C))) attribute, since that does the wrong thing for function templates and
varargs functions.

In passing, fix a grammar error in the diagnostic, a crash if
__attribute__((nonnull(N))) is applied to a varargs function,
a bug where the same null argument could be diagnosed multiple
times if there were multiple nonnull attributes referring to it,
and a bug where nonnull attributes would not be accumulated correctly
across redeclarations.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216520 91177308-0d34-0410-b5e6-96231b3b80d8