[Serialization] Move (Module)Status and validateSerializedAST into a namespace.

Also into a separate file.

Before (swift/Serialization/SerializedModuleLoader.h):
  ModuleStatus
  SerializedModuleLoader::ValidationInfo
  SerializedModuleLoader::ExtendedValidationInfo
  SerializedModuleLoader::isSerializedAST
  SerializedModuleLoader::validateSerializedAST

After (swift/Serialization/Validation.h):
  serialization::Status
  serialization::ValidationInfo
  serialization::ExtendedValidationInfo
  serialization::isSerializedAST
  serialization::validateSerializedAST

No functionality change, just a lot of renaming and a bit of reorganizing.

Swift SVN r25226

lib/Serialization/SerializedSILLoader.cpp

@@ -11,9 +11,10 @@
 //===----------------------------------------------------------------------===//
 
 #define DEBUG_TYPE "serialized-sil-loader"
+#include "swift/Serialization/SerializedSILLoader.h"
 #include "DeserializeSIL.h"
 #include "swift/Serialization/ModuleFile.h"
-#include "swift/Serialization/SerializedSILLoader.h"
+#include "swift/Serialization/SerializedModuleLoader.h"
 #include "swift/SIL/SILModule.h"
 #include "llvm/Support/Debug.h"
 
@@ -24,6 +25,7 @@ SerializedSILLoader::SerializedSILLoader(ASTContext &Ctx,
                                          Callback *callback) {
 
   // Get a list of SerializedModules from ASTContext.
+  // FIXME: Iterating over LoadedModules is not a good way to do this.
   for (auto &Entry : Ctx.LoadedModules) {
     for (auto File : Entry.second->getFiles()) {
       if (auto LoadedAST = dyn_cast<SerializedASTFile>(File)) {