build: work around issue with Boost <= 1.80 and Clang >= 18

Our current minimum supported Boost is `1.73.0`. However, when compiling
with Boost `1.74.0` (Debian Stable), using Clang `18`, compilation fails
with:
```bash
In file included from /usr/include/boost/mpl/integral_c.hpp:32:
/usr/include/boost/mpl/aux_/integral_wrapper.hpp:73:31: error: integer value -1 is outside the valid range of values [0, 3] for the enumeration type 'udt_builtin_mixture_enum' [-Wenum-constexpr-conversion]
   73 |     typedef AUX_WRAPPER_INST( BOOST_MPL_AUX_STATIC_CAST(AUX_WRAPPER_VALUE_TYPE, (value - 1)) ) prior;
      |                               ^
/usr/include/boost/mpl/aux_/static_cast.hpp:24:47: note: expanded from macro 'BOOST_MPL_AUX_STATIC_CAST'
   24 | #   define BOOST_MPL_AUX_STATIC_CAST(T, expr) static_cast<T>(expr)
      |                                               ^
In file included from ../../../src/test/validation_chainstatemanager_tests.cpp:8:
In file included from ../../../src/node/chainstatemanager_args.h:9:
In file included from ../../../src/validation.h:28:
In file included from ../../../src/txmempool.h:26:
In file included from /usr/include/boost/multi_index/hashed_index.hpp:38:
In file included from /usr/include/boost/multi_index/detail/node_handle.hpp:22:
In file included from /usr/include/boost/multi_index_container_fwd.hpp:18:
In file included from /usr/include/boost/multi_index/indexed_by.hpp:17:
In file included from /usr/include/boost/mpl/vector.hpp:36:
In file included from /usr/include/boost/mpl/vector/vector20.hpp:18:
In file included from /usr/include/boost/mpl/vector/vector10.hpp:18:
In file included from /usr/include/boost/mpl/vector/vector0.hpp:24:
In file included from /usr/include/boost/mpl/vector/aux_/clear.hpp:18:
In file included from /usr/include/boost/mpl/vector/aux_/vector0.hpp:22:
In file included from /usr/include/boost/mpl/vector/aux_/iterator.hpp:19:
In file included from /usr/include/boost/mpl/plus.hpp:19:
In file included from /usr/include/boost/mpl/aux_/arithmetic_op.hpp:17:
In file included from /usr/include/boost/mpl/integral_c.hpp:32:
/usr/include/boost/mpl/aux_/integral_wrapper.hpp:73:31: error: integer value -1 is outside the valid range of values [0, 3] for the enumeration type 'int_float_mixture_enum' [-Wenum-constexpr-conversion]
/usr/include/boost/mpl/aux_/static_cast.hpp:24:47: note: expanded from macro 'BOOST_MPL_AUX_STATIC_CAST'
   24 | #   define BOOST_MPL_AUX_STATIC_CAST(T, expr) static_cast<T>(expr)
      |                                               ^
2 errors generated.
```

Work around this issue by ignoring this diagnostic for this include.
I did attempt to just downgrade the error into a warning, but that did
not seem to work.

See bitcoin#30751 for further
discussion.

Author: fanquake

src/txmempool.h

@@ -23,7 +23,17 @@
 #include <util/result.h>
 #include <util/feefrac.h>
 
+// This works around a bug in Boost <= 1.80.0 when using Clang >=18.
+// See https://github.com/bitcoin/bitcoin/issues/30751.
+#if defined(__clang__)
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wenum-constexpr-conversion"
+#endif
 #include <boost/multi_index/hashed_index.hpp>
+#if defined(__clang__)
+#pragma clang diagnostic pop
+#endif
+
 #include <boost/multi_index/identity.hpp>
 #include <boost/multi_index/indexed_by.hpp>
 #include <boost/multi_index/ordered_index.hpp>