Added GraphTraits to source-level CFGs (CFG and CFGBlock) to allow

Added GraphTraits to source-level CFGs (CFG and CFGBlock) to allow
(LLVM-provided) graph algorithms such as DFS and graph visualization
to work effortless on source-level CFGs.

Further cleanup on pretty printing of CFGs.  CFGBlock::dump and
CFGBlock::print now take the parent CFG as an argument.  This allows
CFGBlocks to print their own appropriate label indicating whether or
not they are the Entry/Exit/IndirectGotoBlock without the CFG::print
routine doing it instead.

Added Graphviz visualization for CFGs: CFG::viewCFG.  This employs the
GraphTraits just implemented.

Added "-view-cfg" mode the to clang driver.  This is identical to
"-dump-cfg" except that it calls Graphviz to visualize the CFGs
instead of dumping them to the terminal.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@41580 91177308-0d34-0410-b5e6-96231b3b80d8