Wrap local vars in std::move (Clang 7 build fix)

[karkhaz]

This commit fixes #3238, a warning emitted by clang 7.0:

java_bytecode_convert_method.cpp:2924:10: error: local variable
'block' will be copied despite being returned by name
[-Werror,-Wreturn-std-move]
  return block;
         ^~~~~
java_bytecode_convert_method.cpp:2924:10: note: call 'std::move'
explicitly to avoid copying
  return block;
         ^~~~~
         std::move(block)

The fix is to wrap all such returned local variables in a call to
std::move().

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[smowton]

Did you arrive at this particular list of return sites by a regex or similar match, or is this exactly equal to the places where Clang 7 complains?

[karkhaz]

It is exactly equal to the places where Clang 7 complains. I don't think a simple regex could deal with this, it's context-sensitive, the variables need to be local.

[smowton]

Great -- it sounds like we're not adding this in cases where return-value optimisation would apply in that case.

[karkhaz]

Yes, this is a pretty cool warning. I'm not sure who is in charge of the CI, but perhaps it's worth adding Clang 7 at some point? I don't mind posting PRs for any more of these that people introduce in the meantime, if nobody else is using Clang 7 yet, but it might be worth doing at some point.

[allredj]

Passed Diffblue compatibility checks (cbmc commit: 72bfc35).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/89432202