[analyzer] Clean up the printing of FieldRegions for leaks.

Unfortunately, generalized region printing is very difficult:
- ElementRegions are used both for casting and as actual elements.
- Accessing values through a pointer means going through an intermediate
  SymbolRegionValue; symbolic regions are untyped.
- Referring to implicitly-defined variables like 'this' and 'self' could be
  very confusing if they come from another stack frame.

We fall back to simply not printing the region name if we can't be sure it
will print well. This will allow us to improve in the future.

llvm-svn: 161512