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/driver/dex_compilation_unit.cc b/compiler/driver/dex_compilation_unit.cc
index e5a6f0e..0d0f074 100644
--- a/compiler/driver/dex_compilation_unit.cc
+++ b/compiler/driver/dex_compilation_unit.cc
@@ -30,7 +30,7 @@
 DexCompilationUnit::DexCompilationUnit(Handle<mirror::ClassLoader> class_loader,
                                        ClassLinker* class_linker,
                                        const DexFile& dex_file,
-                                       const DexFile::CodeItem* code_item,
+                                       const dex::CodeItem* code_item,
                                        uint16_t class_def_idx,
                                        uint32_t method_idx,
                                        uint32_t access_flags,