type-function.h: Fix CustomBuildField overload #181

1. 
   ryanofsky commented at 7:58 pm on June 3, 2025: collaborator

   Fix std::function CustomBuildField overload which is incompatible with a recent change in 3a96cdc18f2d1ca202fbc91551f27097fd7ec7f6 from #172 which changed generated IPC client code to pass it an rvalue std::function reference instead of an lvalue reference.

   Motivation for this change is to avoid a build error in https://github.com/bitcoin/bitcoin/pull/29409, when rebased on top of https://github.com/bitcoin/bitcoin/pull/32641 which includes #172.

2. type-function.h: Fix CustomBuildField overload

   Fix std::function CustomBuildField overload which is incompatible with a recent
   change in 3a96cdc18f2d1ca202fbc91551f27097fd7ec7f6 from
   https://github.com/bitcoin-core/libmultiprocess/pull/172 which changed
   generated IPC client code to pass it an rvalue std::function reference instead
   of an lvalue reference.
   
   There was no test coverage for the type-function.h header earlier but the next
   commit adds a test which would have caught the problem in the CustomBuildField
   declaration.
   
   Motivation for this change is to avoid a build error in
   https://github.com/bitcoin/bitcoin/pull/29409 when rebased on top of
   https://github.com/bitcoin/bitcoin/pull/32641 which includes
   https://github.com/bitcoin-core/libmultiprocess/pull/172

   8b96229da5
3. test: Add coverage for type-function.h 688140b1df
4. TheCharlatan approved
5. 
   TheCharlatan commented at 8:22 am on June 11, 2025: collaborator

   ACK 688140b1dffc78a97a5a16f62b540a55824f4091
6. ryanofsky merged this on Jun 11, 2025
7. ryanofsky closed this on Jun 11, 2025

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8. ryanofsky referenced this in commit 07dda23f6b on Jun 13, 2025
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Contributors
ryanofsky TheCharlatan