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

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@142693 91177308-0d34-0410-b5e6-96231b3b80d8