Added an (optional) explicit "Label" statement to CFGBlock. If a

block begins with a label or case statement, CFGBlock::getLabel() will
return the corresponding LabelStmt/CaseStmt/DefaultStmt.

LabelStmts and SwitchCases no longer appear within the "executable"
statements of a CFGBlock.

More cleanups on visualization/pretty-printing of CFGs (less verbose).

llvm-svn: 41585