[ADT] Add some documentation for GraphTraits.

[timshen]

Summary

Add some context for GraphTraits.

[dberris]

llvm/include/llvm/ADT/GraphTraits.h

28–30

Typo: "facncier"

Also, this could be a simpler sentence without "value judgments". Something like:

// This template evolved from supporting `BasicBlock` to also later supporting
// more complex types (e.g. CFG and DomTree).

32–35

I suggest simplifying as well, maybe like:

// GraphTraits can be used to create a view over a graph interpreting it
// differently without requiring a copy of the original graph. This could
// be used for working with NodeRef's and loop bodies (see LoopBodyTraits
// for one example).

[asbirlea]

+1 on both dberris's comments.

Thank you for the additional documentation!

[timshen]

Update based on comments.

[dberris]

LGTM -- thanks for the additional documentation!

llvm/include/llvm/ADT/GraphTraits.h

 
 #include "llvm/ADT/iterator_range.h"
 
 namespace llvm {
 
 // GraphTraits - This class should be specialized by different graph types...
 // which is why the default version is empty.
 //
+// This template evolved from supporting `BasicBlock` to also later supporting
+// more complex types (e.g. CFG and DomTree).
+//
+// GraphTraits can be used to create a view over a graph interpreting it
+// differently without requiring a copy of the original graph. This could
+// be achieved by carrying more data in NodeRef. See LoopBodyTraits for one
+// example.
 template<class GraphType>
 struct GraphTraits {
   // Elements to provide:
 
   // typedef NodeRef           - Type of Node token in the graph, which should
   //                             be cheap to copy.
   // typedef ChildIteratorType - Type used to iterate over children in graph,
   //                             dereference to a NodeRef.