Fix behavior of [[gnu::]] function attributes. Per g++'s behavior, these

attributes appertain to a declaration, even though they would be much more
naturally modelled as appertaining to a function type. Previously, we would
try to distribute them from the declarator to the function type, then
reject them for being at an incorrect location. Now, we just distribute them
as far as the declarator; the existing attribute handling code can actually
apply them there just fine.

llvm-svn: 172504