Place constructors and destructors into the DeclContext of the class,

just like all other members, and remove the special variables in
CXXRecordDecl to store them. This eliminates a lot of special-case
code for constructors and destructors, including
ActOnConstructor/ActOnDeclarator and special lookup rules in
LookupDecl. The result is far more uniform and manageable.

Diagnose the redeclaration of member functions.

llvm-svn: 61048