[analyzer] Mark heap-based symbolic regions in debug dumps.

They are now printed as HeapSymRegion{$x} in order to discriminate between that
and regular SymRegion{$x}, which are two different regions, having different
parent reginos (memory spaces) - HeapSpaceRegion and UnknownSpaceRegion
respectively.

Differential Revision: https://reviews.llvm.org/D40793

llvm-svn: 319793