llvm-cov: Added edge struct for traversal in block.

Added GCOVEdge which are simple structs owned by the GCOVFunction that
stores the source and destination GCOVBlocks, as well as the counts.
Changed GCOVBlocks so that it stores a vector of source GCOVEdges and a
vector of destination GCOVEdges, rather than just the block number.

Storing the block number was only useful for knowing the number of edges
and for debug info. Using a struct is useful for traversing the edges,
especially back edges which may be needed later.