Model temporary destructors from logical operators with known values.

If the truth value of a LHS is known, we can build the knowledge whether
a temporary destructor is executed or not into the CFG. This is needed
by the return type analysis.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@215118 91177308-0d34-0410-b5e6-96231b3b80d8