Add assert and fix comment

Author: royitaqi

lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARFDebugMap.cpp

@@ -246,6 +246,7 @@ llvm::StringRef SymbolFileDWARFDebugMap::GetPluginDescriptionStatic() {
 }
 
 SymbolFile *SymbolFileDWARFDebugMap::CreateInstance(ObjectFileSP objfile_sp) {
+  assert(objfile_sp);
   // Don't create a debug map if the object file isn't a Mach-O.
   if (!objfile_sp->GetArchitecture().GetTriple().isAppleMachO())
     return nullptr;

lldb/unittests/SymbolFile/DWARF/SymbolFileDWARFDebugMapTests.cpp

@@ -73,7 +73,7 @@ TEST_F(SymbolFileDWARFDebugMapTests, CreateInstanceReturnNonNullForMachOFile) {
 ...
 )";
 
-  // Set up
+  // Perform setup.
   llvm::Expected<TestFile> file = TestFile::fromYaml(yamldata);
   EXPECT_THAT_EXPECTED(file, llvm::Succeeded());
   auto module_sp = std::make_shared<Module>(file->moduleSpec());
@@ -139,7 +139,7 @@ TEST_F(SymbolFileDWARFDebugMapTests, CreateInstanceReturnNullForNonMachOFile) {
           Length:          0
 )";
 
-  // Set up
+  // Perform setup.
   llvm::Expected<TestFile> file = TestFile::fromYaml(yamldata);
   EXPECT_THAT_EXPECTED(file, llvm::Succeeded());
   auto module_sp = std::make_shared<Module>(file->moduleSpec());