Workaround attribute ordering issue with kernel only attributes

Workaround attribute ordering issue with kernel only attributes

Placing the attribute after the kernel keyword would incorrectly
reject the attribute, so use the smae workaround that other
kernel only attributes use.

Also add a FIXME because there are two different phrasings now
for the same error, althoug amdgpu_num_[sv]gpr uses a consistent one.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223490 91177308-0d34-0410-b5e6-96231b3b80d8