Deterministic DexCache in heap images.

Preload each slot in dex cache arrays with the candidate
entry with the lowest index.

Also fix a discrepancy where we used to keep an ArtField*
in the DexCache if we kept the declaring class rather than
based on the class referenced in the corresponding FieldId.
This can result in accessing a field through a FieldId with
an unresolved class as demonstrated by the regression test.

Test: 159-app-image-fields
Test: testrunner.py --host
Bug: 30627598
Change-Id: I34735047cb064ff462cd9859e86a61ddf8615ecd
diff --git a/compiler/optimizing/sharpening.cc b/compiler/optimizing/sharpening.cc
index 7bd38c7..eedaf6e 100644
--- a/compiler/optimizing/sharpening.cc
+++ b/compiler/optimizing/sharpening.cc
@@ -259,7 +259,7 @@
     } else if (runtime->UseJitCompilation()) {
       // TODO: Make sure we don't set the "compile PIC" flag for JIT as that's bogus.
       // DCHECK(!codegen_->GetCompilerOptions().GetCompilePic());
-      string = class_linker->LookupString(dex_file, string_index, dex_cache);
+      string = class_linker->LookupString(dex_file, string_index, dex_cache.Get());
       if (string != nullptr) {
         if (runtime->GetHeap()->ObjectIsInBootImageSpace(string)) {
           desired_load_kind = HLoadString::LoadKind::kBootImageAddress;
@@ -271,7 +271,7 @@
       }
     } else {
       // AOT app compilation. Try to lookup the string without allocating if not found.
-      string = class_linker->LookupString(dex_file, string_index, dex_cache);
+      string = class_linker->LookupString(dex_file, string_index, dex_cache.Get());
       if (string != nullptr &&
           runtime->GetHeap()->ObjectIsInBootImageSpace(string) &&
           !codegen_->GetCompilerOptions().GetCompilePic()) {