[CFG] Fix automatic destructors when a member is bound to a reference.

In code like

    const int &x = A().x;

automatic destructor for the object A() lifetime-extended by reference 'x' was
not present in the clang CFG due to ad-hoc pattern-matching in
getReferenceInitTemporaryType().

Re-use skipRValueSubobjectAdjustments() again to find the lifetime-extended
object in the AST and emit the correct destructor.

Lifetime extension through aggregates with references still needs to be covered.

Differential Revision: https://reviews.llvm.org/D44238

llvm-svn: 333941