[analyzer] Clean up the program state map of DanglingInternalBufferChecker.

Symbols are cleaned up from the program state map when they go out of scope.
Memory regions are cleaned up when the corresponding object is destroyed, and
additionally in 'checkDeadSymbols' in case destructor modeling was incomplete.

Differential Revision: https://reviews.llvm.org/D47416

llvm-svn: 334352