[CFG] Provide construction contexts when constructors have cleanups.

Now that we make it possible to query the CFG constructor element to find
information about the construction site, possible cleanup work represented by
ExprWithCleanups should not prevent us from providing this information.

This allows us to have a correct construction context for variables initialized
"by value" via elidable copy-constructors, such as 'i' in

  iterator i = vector.begin();

Differential Revision: https://reviews.llvm.org/D42719

llvm-svn: 324798