Build fails with Clang 7.0 due to local variable copies

[karkhaz]

When building with Clang 7.0, I'm getting thousands of errors like this.

cbmc/src/pointer-analysis/value_set_fivr.cpp:360:10: error: local variable 'od' will be copied despite being returned by name [-Werror,-Wreturn-std-move]
  return od;
         ^~
/home/kareem/doc/cbmc/src/pointer-analysis/value_set_fivr.cpp:360:10: note: call 'std::move' explicitly to avoid copying
  return od;
         ^~
         std::move(od)

I'll fix these soon. Here's the environment:

> clang --version
clang version 7.0.0 (tags/RELEASE_700/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin