Emit complete constructors and destructors as aliases to base constructors

Emit complete constructors and destructors as aliases to base constructors
and destructors when the two entities are semantically identical, i.e. when
the class has no virtual base classes.  We only do this for linkage types
for which aliases are supported, i.e. internal and external, i.e. not linkonce.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@96451 91177308-0d34-0410-b5e6-96231b3b80d8