Move most of class linker to ObjPtr

Return values are still mirror pointer. Fix some failing asserts in
compiler driver and tests.

Bug: 31113334

Test: test-art-host

Change-Id: I4450bf9dfb2541749496b8388616e8aae8488919
diff --git a/compiler/driver/compiler_driver_test.cc b/compiler/driver/compiler_driver_test.cc
index 20f0e5e..845028d 100644
--- a/compiler/driver/compiler_driver_test.cc
+++ b/compiler/driver/compiler_driver_test.cc
@@ -106,7 +106,7 @@
   ScopedObjectAccess soa(Thread::Current());
   ASSERT_TRUE(java_lang_dex_file_ != nullptr);
   const DexFile& dex = *java_lang_dex_file_;
-  mirror::DexCache* dex_cache = class_linker_->FindDexCache(soa.Self(), dex);
+  ObjPtr<mirror::DexCache> dex_cache = class_linker_->FindDexCache(soa.Self(), dex);
   EXPECT_EQ(dex.NumStringIds(), dex_cache->NumStrings());
   for (size_t i = 0; i < dex_cache->NumStrings(); i++) {
     const mirror::String* string = dex_cache->GetResolvedString(i);