[analyzer] exploded-graph-rewriter: Implement manual graph trimming.

When -trim-egraph is unavailable (say, when you're debugging a crash on
a real-world code that takes too long to reduce), it makes sense to view
the untrimmed graph up to the crashing node's predecessor, then dump the ID
(or a pointer) of the node in the attached debugger, and then trim
the dumped graph in order to keep only paths from the root to the node.

The newly added --to flag does exactly that:

$ exploded-graph-rewriter.py ExprEngine.dot --to 0x12229acd0

Multiple nodes can be specified. Stable IDs of nodes can be used
instead of pointers.

Differential Revision: https://reviews.llvm.org/D65345

llvm-svn: 368768