Avoid copies in range-for loops and add a warning to detect them
See title. Fixing these would otherwise be a continuous process, adding the warning should keep them from cropping up. Note that the warning seems to be Clang-only for now.
Some more explanations from the PR comments:
From the pull-request to llvm that added the feature, it warns about: for (const Foo &x : Foos), where the range Foos does not return a copy. This warning will suggest using the non-reference type so the copy is obvious. for (const Foo x : Foos), where the range Foos does return a reference, but is copied into x. This warning will suggest using the reference type to prevent a copy from being made. for (const Bar &x : Foos), where Bar is constructed from Foo. In this case, suggest using the non-reference "const Bar" to indicate a copy is intended to be made, or "const Foo &" to prevent a copy from being made.
Backport of core PR13480
Depends on D3795.
make check ninja check
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D3796