Declare SourceFile::~SourceFile() in header (#26059)

There was an error when including Module.h in an external project with
std::default_delete being instantiated when making a std::unique_ptr.
This change declares destructor in header file and sets it to default in
cpp file.

include/swift/AST/Module.h

@@ -1095,6 +1095,8 @@ public:
              ImplicitModuleImportKind ModImpKind, bool KeepParsedTokens = false,
              bool KeepSyntaxTree = false);
 
+  ~SourceFile();
+
   void addImports(ArrayRef<ImportedModuleDesc> IM);
 
   enum ImportQueryKind {
@@ -1320,7 +1322,7 @@ public:
   }
   
   void markDeclWithOpaqueResultTypeAsValidated(ValueDecl *vd);
-  
+
 private:
 
   /// If not None, the underlying vector should contain tokens of this source file.