[Serialization] Retire "shadowed module" in favor of "underlying" (#24711)

Similar to 517f5d6b6a, the "shadowed" terminology didn't end up
describing the most common use of the feature; there is pretty much no
intended case where a Swift module shadows a Clang module without also
re-exporting it. Switch to "underlying", which was already in use in a
few places, and which better parallels "overlay".

No intended functionality change.

include/swift/Serialization/ModuleFile.h

@@ -57,8 +57,8 @@ class ModuleFile
   /// A reference back to the AST representation of the file.
   FileUnit *FileContext = nullptr;
 
-  /// The module shadowed by this module, if any.
-  ModuleDecl *ShadowedModule = nullptr;
+  /// The module that this module is an overlay of, if any.
+  ModuleDecl *UnderlyingModule = nullptr;
 
   /// The module file data.
   std::unique_ptr<llvm::MemoryBuffer> ModuleInputBuffer;
@@ -702,8 +702,8 @@ public:
     return Dependencies;
   }
 
-  /// The module shadowed by this module, if any.
-  ModuleDecl *getShadowedModule() const { return ShadowedModule; }
+  /// The module that this module is an overlay for, if any.
+  ModuleDecl *getUnderlyingModule() const { return UnderlyingModule; }
 
   /// Searches the module's top-level decls for the given identifier.
   void lookupValue(DeclName name, SmallVectorImpl<ValueDecl*> &results);