[Conformance checking] Put associated type inference into its own file.

Factor associated type inference out into its own separate source file, so
it's easier to maintain on its own.

Author: DougGregor

lib/Sema/CMakeLists.txt

@@ -48,6 +48,7 @@ add_swift_library(swiftSema STATIC
   TypeCheckNameLookup.cpp
   TypeCheckPattern.cpp
   TypeCheckProtocol.cpp
+  TypeCheckProtocolInference.cpp
   TypeCheckREPL.cpp
   TypeCheckRequest.cpp
   TypeCheckStmt.cpp

lib/Sema/DerivedConformances.h

@@ -22,11 +22,14 @@
 
 namespace swift {
   class Decl;
+  class DeclRefExpr;
   class FuncDecl;
   class NominalTypeDecl;
+  class PatternBindingDecl;
   class Type;
   class TypeChecker;
   class ValueDecl;
+  class VarDecl;
 
 namespace DerivedConformance {