Handle constructors and destructors a bit more uniformly in CodeGen.

There were code paths that are duplicated for constructors and destructors just
because we have both CXXCtorType and CXXDtorsTypes.

This patch introduces an unified enum and reduces code deplication a bit.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217383 91177308-0d34-0410-b5e6-96231b3b80d8