Clean up AtomicDexRefMap

Make ClassReference, TypeReference, and MethodReference extend
DexFileReference. This enables using all of these types as the key
for AtomicDexRefMap.

Test: test-art-host
Bug: 63851220
Bug: 63756964

Change-Id: Ida3c94cadb53272cb5057e5cebc5971c1ab4d366
diff --git a/compiler/optimizing/code_generator_mips.cc b/compiler/optimizing/code_generator_mips.cc
index f0ef007..9db2bd3 100644
--- a/compiler/optimizing/code_generator_mips.cc
+++ b/compiler/optimizing/code_generator_mips.cc
@@ -1683,7 +1683,7 @@
     MethodReference target_method,
     const PcRelativePatchInfo* info_high) {
   return NewPcRelativePatch(*target_method.dex_file,
-                            target_method.dex_method_index,
+                            target_method.index,
                             info_high,
                             &pc_relative_method_patches_);
 }
@@ -1692,7 +1692,7 @@
     MethodReference target_method,
     const PcRelativePatchInfo* info_high) {
   return NewPcRelativePatch(*target_method.dex_file,
-                            target_method.dex_method_index,
+                            target_method.index,
                             info_high,
                             &method_bss_entry_patches_);
 }