[CTU] Add asserts to protect invariants

[a_sidorin]

More assertions are always good.

[xazax.hun]

lib/CrossTU/CrossTranslationUnit.cpp

249

I wonder if this actually is true. While it is not supported now, we could/might use this to import/merge attributes even without definition.
But of course, I have no hard feelings about having this assert now and remove it later on once other use cases are actually supported.

lib/CrossTU/CrossTranslationUnit.cpp

CrossTranslationUnitContext::findFunctionInDeclContext(const DeclContext *DC,
   }
   return nullptr;
 }
 
 llvm::Expected<const FunctionDecl *>
 CrossTranslationUnitContext::getCrossTUDefinition(const FunctionDecl *FD,
                                                   StringRef CrossTUDir,
                                                   StringRef IndexName) {
+  assert(FD && "FD is missing, bad call to this function!");
   assert(!FD->hasBody() && "FD has a definition in current translation unit!");
   const std::string LookupFnName = getLookupName(FD);
   if (LookupFnName.empty())
     return llvm::make_error<IndexError>(
         index_error_code::failed_to_generate_usr);
   llvm::Expected<ASTUnit *> ASTUnitOrError =
       loadExternalAST(LookupFnName, CrossTUDir, IndexName);
   if (!ASTUnitOrError)
llvm::Expected<ASTUnit *> CrossTranslationUnitContext::loadExternalAST(
   } else {
     Unit = FnUnitCacheEntry->second;
   }
   return Unit;
 }
 
 llvm::Expected<const FunctionDecl *>
 CrossTranslationUnitContext::importDefinition(const FunctionDecl *FD) {
+  assert(FD->hasBody() && "Functions to be imported should have body.");
+
   ASTImporter &Importer = getOrCreateASTImporter(FD->getASTContext());
   auto *ToDecl =
       cast<FunctionDecl>(Importer.Import(const_cast<FunctionDecl *>(FD)));
   assert(ToDecl->hasBody());
   assert(FD->hasBody() && "Functions already imported should have body.");
   return ToDecl;
 }