AST: fix build since r152060

The declarations of the operators no longer matched.
The definitions in ASTContext.h had 'throw()' removed,
but it was still present in Attr.h.

Somehow the buildbots missed this. clang merely warns about
a missing 'throw()' specification and suggested a Fix-It adding
it back, but gcc 4.5 is not so forgiving and gives an error.

llvm-svn: 152167