Place constructors and destructors into the DeclContext of the class,

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@61048 91177308-0d34-0410-b5e6-96231b3b80d8