[Doc parsing] This patch searches overridden objc/c++

[Doc parsing] This patch searches overridden objc/c++
methods looking for documentation on a particular base
class inherited by any method that overrides the base class.
In case of redeclaration, as when objc method is defined
in the implementation, it also looks up for documentation
in class/class extension being redeclared.

llvm-svn: 165643