Introduce reference to avoid unneeded copy.

[NlightNFotis]

Previously this broke the build on macOS, because the compiler was complaining about creating a copy of a const std::pair - which
registered as an error (because of -Werror) - on Apple Clang 12.0.0.

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[hannes-steffenhagen-diffblue]

LGTM. Reports like this make me wonder though if there's a way to tell compilers you’re intentionally making a copy. I guess you wouldn't do that too often and having an extra assignment statements in those cases should be sufficient to signal this to developers and compilers?

[codecov]

Codecov Report

Merging #5524 into develop will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #5524   +/-   ##
========================================
  Coverage    68.40%   68.40%           
========================================
  Files         1187     1187           
  Lines        98090    98090           
========================================
  Hits         67102    67102           
  Misses       30988    30988           

Flag	Coverage Δ
#cproversmt2	42.97% <ø> (ø)
#regression	65.55% <100.00%> (ø)
#unit	32.25% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
jbmc/src/java_bytecode/lambda_synthesis.cpp	97.96% <100.00%> (ø)

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