ART: Move dex structs into own header

Separating out the structs from DexFile allows them to be forward-
declared, which reduces the need to include the dex_file header.

Bug: 119869270
Test: m
Change-Id: I32dde5a632884bca7435cd584b4a81883de2e7b4
diff --git a/compiler/common_compiler_test.cc b/compiler/common_compiler_test.cc
index 72afb98..07c73c9 100644
--- a/compiler/common_compiler_test.cc
+++ b/compiler/common_compiler_test.cc
@@ -264,7 +264,7 @@
     Handle<mirror::DexCache> dex_cache(hs.NewHandle(method->GetDexCache()));
     Handle<mirror::ClassLoader> h_class_loader = hs.NewHandle(
         self->DecodeJObject(class_loader)->AsClassLoader());
-    const DexFile::CodeItem* code_item = dex_file->GetCodeItem(method->GetCodeItemOffset());
+    const dex::CodeItem* code_item = dex_file->GetCodeItem(method->GetCodeItemOffset());
 
     std::vector<const DexFile*> dex_files;
     dex_files.push_back(dex_file);