Store a copy of MethodDebugInfo in ElfWriter

Bug: http://b/62418787

Store a copy of the debug info instead of a reference, since this is
written in the background by a separate thread.

Test: make -j36 PRODUCT-aosp_x86_64-eng dist checkbuild \
          ASAN_OPTIONS=detect_leaks=0 SANITIZE_HOST=address \
          FORCE_BUILD_LLVM_COMPONENTS=true
      doesn't have dex2oat asan failures that are otherwise seen.

Change-Id: I2599f5b607f956191db01924730a2e98421b5d77