Add ELF index to art::compiler_llvm::CompilationUnit.

We need an ELF index to remember which ELF image contains
the CompiledMethod or CompiledInvokeStub.

(cherry picked from commit df612fb98fabeec2fa7277c7ab332589fa38283c)

Change-Id: Ic8a4aca7d030685e156c9f3fe632bf45a5c4d02b
diff --git a/src/compiler_llvm/compilation_unit.h b/src/compiler_llvm/compilation_unit.h
index a0882bb..4a23095 100644
--- a/src/compiler_llvm/compilation_unit.h
+++ b/src/compiler_llvm/compilation_unit.h
@@ -36,10 +36,14 @@
 
 class CompilationUnit {
  public:
-  CompilationUnit(InstructionSet insn_set);
+  CompilationUnit(InstructionSet insn_set, size_t elf_idx);
 
   ~CompilationUnit();
 
+  size_t GetElfIndex() const {
+    return elf_idx_;
+  }
+
   InstructionSet GetInstructionSet() const {
     return insn_set_;
   }
@@ -98,6 +102,7 @@
 
  private:
   InstructionSet insn_set_;
+  const size_t elf_idx_;
 
   UniquePtr<llvm::LLVMContext> context_;
   UniquePtr<IRBuilder> irb_;