c++: support gcc's application of weak attribute on

class declaration which forces any such class and any 
class that inherits from such a class to have their 
typeinfo symbols be marked as weak.
// rdar://10246395
A    test/CodeGenCXX/weak-extern-typeinfo.cpp
M    lib/Sema/SemaDeclCXX.cpp
M    lib/Sema/SemaDeclAttr.cpp
M    lib/CodeGen/CGRTTI.cpp

llvm-svn: 142693