build: add kernel-specific warnings

In some cases, we'll want to be more aggressive or care about different things
when building the kernel. In this case, a warning is added for symbols which
may be duplicated between the kernel and downstream users.

A few caveats for now:
- This warning is introduced in clang 21, which is not yet released
- The warning doesn't actually show unless the CXX_VISIBILITY_PRESET hack is
  disabled in CMake, which won't happen for a while.

So this warning is not helpful for CI at the moment, but it's possible to use
it manually (as I have for this PR) by disabling the hack locally until then.

src/kernel/CMakeLists.txt

@@ -78,9 +78,25 @@ add_library(bitcoinkernel
   ../validationinterface.cpp
   ../versionbits.cpp
 )
+
+# Compiler warnings that apply only to the kernel and its dependencies.
+# These can be more strict and/or warnings that only apply to shared
+# libs.
+add_library(kernel_warn_interface INTERFACE)
+if(MSVC)
+else()
+  try_append_cxx_flags("-Wunique-object-duplication" TARGET kernel_warn_interface SKIP_LINK)
+endif()
+
+# Also manually apply the warnings to the kernel's internal dependencies
+target_link_libraries(bitcoin_clientversion PRIVATE kernel_warn_interface)
+target_link_libraries(bitcoin_crypto PRIVATE kernel_warn_interface)
+target_link_libraries(leveldb PRIVATE kernel_warn_interface)
+
 target_link_libraries(bitcoinkernel
   PRIVATE
     core_interface
+    kernel_warn_interface
     bitcoin_clientversion
     bitcoin_crypto
     leveldb