Serialization: Fix crash when deserializing a generic parameter belonging to a typealias

The next patch creates a situation where we serialize a reference
to a TypeAliasDecl's GenericParamTypeDecl, which references the
inner DeclContext of the TypeAliasDecl itself. This was not being
deserialized properly, triggering assertions.

Author: slavapestov

include/swift/Serialization/ModuleFormat.h

@@ -760,6 +760,7 @@ namespace decls_block {
     TypeIDField, // interface type
     BCFixed<1>,  // implicit flag
     AccessibilityKindField // accessibility
+    // Trailed by generic parameters (if any).
   >;
 
   using GenericTypeParamDeclLayout = BCRecordLayout<

lib/Serialization/Deserialization.cpp

@@ -1556,6 +1556,8 @@ DeclContext *ModuleFile::getDeclContext(DeclContextID DCID) {
     declContextOrOffset = AFD;
   } else if (auto SD = dyn_cast<SubscriptDecl>(D)) {
     declContextOrOffset = SD;
+  } else if (auto TAD = dyn_cast<TypeAliasDecl>(D)) {
+    declContextOrOffset = TAD;
   } else {
     llvm_unreachable("Unknown Decl : DeclContext kind");
   }