Teach AttachPreviousImpl to inherit MSInheritanceAttr attribute

This commit teaches ASTDeclReader::attachPreviousDecl to successfully merge
two Decl's when one contains an inheritable attribute like the
MSInheritanceAttr. Usually, attributes that are needed to be present along the
redeclaration chain are attached during ASTReading from
ASTDeclReader::attachPreviousDecl, but no such thing is done for inheritable
attributes. Currently, only the logic for merging MSInheritanceAttr is
provided.