[analyzer] Support temporaries conjured by conservatively evaluated functions.

Properly perform destruction and lifetime extension of such temporaries.

C++ object-type return values of conservatively evaluated functions are now
represented as compound values of well-defined temporary object regions. The
function creates a region that represents the temporary object and will later
be used for destruction or materialization, invalidates it, and returns the
invalidated compound value of the object.

Differential Revision: https://reviews.llvm.org/D44131

llvm-svn: 327348