[IR] Remove forward declaration of GraphTraits from Type.h

[foad]

Summary

This has been unnecessary since r352353 removed GraphTraits
specializations for Type, except that a couple of other headers were
accidentally relying on this declaration.

[dblaikie]

Sounds OK - though might be worth considering #including "GraphTraits.h" rather than the forward declaration - makes maintenance a bit easier later on, if default template arguments were added to GraphTraits or other changes are made.

[foad]

though might be worth considering #including "GraphTraits.h" rather than the forward declaration - makes maintenance a bit easier later on, if default template arguments were added to GraphTraits or other changes are made.

I considered and rejected it. I think the LLVM project as a whole prefers forward declarations to #includes, for the sake of build time.

llvm/include/llvm/Analysis/LazyCallGraph.h

 #include "llvm/Support/raw_ostream.h"
 #include <cassert>
 #include <iterator>
 #include <string>
 #include <utility>
 
 namespace llvm {
 
+template <class GraphType> struct GraphTraits;
 class Module;
 class Value;
 
 /// A lazily constructed view of the call graph of a module.
 ///
 /// With the edges of this graph, the motivating constraint that we are
 /// attempting to maintain is that function-local optimization, CGSCC-local
 /// optimizations, and optimizations transforming a pair of functions connected

llvm/include/llvm/IR/ModuleSummaryIndex.h

 #include <memory>
 #include <set>
 #include <string>
 #include <utility>
 #include <vector>
 
 namespace llvm {
 
+template <class GraphType> struct GraphTraits;
+
 namespace yaml {
 
 template <typename T> struct MappingTraits;
 
 } // end namespace yaml
 
 /// Class to accumulate and hold information about a callee.
 struct CalleeInfo {

llvm/include/llvm/IR/Type.h

 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/TypeSize.h"
 #include <cassert>
 #include <cstdint>
 #include <iterator>
 
 namespace llvm {
 
-template<class GraphType> struct GraphTraits;
 class IntegerType;
 class LLVMContext;
 class PointerType;
 class raw_ostream;
 class StringRef;
 
 /// The instances of the Type class are immutable: once they are created,
 /// they are never changed.  Also note that only one instance of a particular