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/optimizing/nodes.cc b/compiler/optimizing/nodes.cc
index e9a2f96..f7c16d1 100644
--- a/compiler/optimizing/nodes.cc
+++ b/compiler/optimizing/nodes.cc
@@ -689,7 +689,7 @@
 }
 
 const char* HGraph::GetMethodName() const {
-  const DexFile::MethodId& method_id = dex_file_.GetMethodId(method_idx_);
+  const dex::MethodId& method_id = dex_file_.GetMethodId(method_idx_);
   return dex_file_.GetMethodName(method_id);
 }