Process attributes in the order they appear in the source code. This make clang

match gcc behavior for two conflicting visibilities in the same decl. It also
makes handling of dllimport/dllexport more natural.

As a bonus we now warn on the dllimport in

void __attribute__((dllimport)) foo13();
void __attribute__((dllexport)) foo13();

as does gcc.

llvm-svn: 156343