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CodeGen's only dependency on ErrorCode was one MakeInstruction()
overload, which can be removed by making the caller responsible for
converting the ErrorCode to a BPF_RET'able value. Fortunately, there
are only two uses of this function:

1. SandboxBPF::RetExpression() which is easily fixed by simply calling
ErrorCode::err() to get the ErrorCode's error value.

2. codegen_unittest.cc which is merely testing the BPF code generator,
and so any return values will do: no need to be seccomp-bpf return
values... so just replace them with simple integer values.  (While
here, change a few bare "BPF_RET" arguments to "BPF_RET + BPF_K" just
to be consistent.)

After this CL, CodeGen focuses solely on assembling programs for the
abstract BPF machine, while higher-level abstractions (primarily
SandboxBPF) are responsible for using it in a way that's semantically
meaningful for seccomp-bpf.

BUG=414363

Review URL: https://codereview.chromium.org/576673003

Cr-Commit-Position: refs/heads/master@{#295192}