[ScanDependency] Move binary module validation into scanner

Improve swift dependency scanner by validating and selecting dependency
module into scanner. This provides benefits that:
* Build system does not need to schedule interface compilation task if
  the candidate module is picked, it can just use the candidate module
  directly.
* There is no need for forwarding module in the explicit module build.
  Since the build system is coordinating the build, there is no need for
  the forwarding module in the module cache to avoid duplicated work,
* This also correctly supports all the module loading modes in the
  dependency scanner.

This is achieved by only adding validate and up-to-date binary module as
the candidate module for swift interface module dependency. This allows
caching build to construct the correct dependency in the CAS. If there
is a candidate module for the interface module, dependency scanner will
return a binary module dependency in the dependency graph.

The legacy behavior is mostly preserved with a hidden frontend flag
`-no-scanner-module-validation`, while the scanner output is mostly
interchangeable with new scanner behavior with `prefer-interface` module
loading mode except the candidate module will not be returned.

rdar://123711823

include/swift/Serialization/SerializedModuleLoader.h

@@ -16,6 +16,7 @@
 #include "swift/AST/FileUnit.h"
 #include "swift/AST/Module.h"
 #include "swift/AST/ModuleLoader.h"
+#include "swift/AST/SearchPathOptions.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/PrefixMapper.h"
 #include "llvm/TargetParser/Triple.h"
@@ -28,15 +29,6 @@ namespace file_types {
   enum ID : uint8_t;
 }
 
-/// Specifies how to load modules when both a module interface and serialized
-/// AST are present, or whether to disallow one format or the other altogether.
-enum class ModuleLoadingMode {
-  PreferInterface,
-  PreferSerialized,
-  OnlyInterface,
-  OnlySerialized
-};
-
 /// How a dependency should be loaded.
 ///
 /// \sa getTransitiveLoadingBehavior