[analyzer] Handle destructors for the argument to C++ 'delete'.

Now that the CFG includes nodes for the destructors in a delete-expression,
process them in the analyzer using the same common destructor interface
currently used for local, member, and base destructors. Also, check for when
the value is known to be null, in which case no destructor is actually run.

This does not yet handle destructors for deleted /arrays/, which may need
more CFG work. It also causes a slight regression in the location of
double delete warnings; the double delete is detected at the destructor
call, which is implicit, and so is reported on the first access within the
destructor instead of at the 'delete' statement. This will be fixed soon.

Patch by Karthik Bhat!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191381 91177308-0d34-0410-b5e6-96231b3b80d8
PR17359: Fix off-by-one OOB on _Pragma("") and an unescaping bug

Previously the code would reduce a run of backslashes to a single
backslash, and now it will properly leave behind every other backslash.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191382 91177308-0d34-0410-b5e6-96231b3b80d8