Merge bitcoin/bitcoin#29494: build: Assume HAVE_CONFIG_H, Add IWYU pragma keep to bitcoin-config.h includes

fa09451f8e6799682d7e7c863f25334fd1c7dce3 Add lint check for bitcoin-config.h include IWYU pragma (MarcoFalke)
dddd40ba8267dea11a3eb03d5cf8b51dbb99be5d scripted-diff: Add IWYU pragma keep to bitcoin-config.h includes (MarcoFalke)

Pull request description:

  The `bitcoin-config.h` includes have issues:

  * The header is incompatible with iwyu, because symbols may be defined or not defined. So the `IWYU pragma: keep` is needed to keep the include when a symbol is not defined on a platform. Compare the previous discussion in https://github.com/bitcoin/bitcoin/pull/29408#issuecomment-1948959711
  * Guarding the includes by `HAVE_CONFIG_H` is verbose and brittle. Now that all build config dependencies have been removed from low level headers, the benefits are questionable, and the guard can be removed. The linter could also be tricked by guarding the include by `#if defined(HAVE_C0NFIG_H)` (`O` replaced by `0`). Compare the previous discussion in https://github.com/bitcoin/bitcoin/pull/29404#discussion_r1483189853 .

ACKs for top commit:
  achow101:
    ACK fa09451f8e6799682d7e7c863f25334fd1c7dce3
  TheCharlatan:
    ACK fa09451f8e6799682d7e7c863f25334fd1c7dce3
  hebasto:
    re-ACK fa09451f8e6799682d7e7c863f25334fd1c7dce3, only rebased since my recent [review](https://github.com/bitcoin/bitcoin/pull/29494#pullrequestreview-2028864535) (`timedata.cpp` removed in https://github.com/bitcoin/bitcoin/pull/29623).

Tree-SHA512: 47cb973f7f24bc625acc4e78683371863675d186780236d55d886cf4130e05a78bb04f1d731aae7088313b8e963a9677cc77cf518187dbd99d776f6421ca9b52

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