tools: use LibXml2::LibXml2 to link against

Adjust the build rules to use the `LibXml2::LibXml2` target rather
than use the explicit include paths and link flags. This allows us to
track additional dependencies (implicit linked libraries) as well as
properly propagate the include paths and library search paths.

Cherrypick commit swiftlang#77848

Author: hjyamauchi

tools/swift-ide-test/CMakeLists.txt

@@ -17,12 +17,8 @@ target_link_libraries(swift-ide-test
 
 # If libxml2 is available, make it available for swift-ide-test.
 if(LLVM_ENABLE_LIBXML2)
-  include_directories(SYSTEM ${LIBXML2_INCLUDE_DIR})
-  target_link_libraries(swift-ide-test PRIVATE ${LIBXML2_LIBRARIES})
+  target_link_libraries(swift-ide-test PRIVATE LibXml2::LibXml2)
   target_compile_definitions(swift-ide-test PRIVATE SWIFT_HAVE_LIBXML=1)
-  if("${CMAKE_HOST_SYSTEM_NAME}" STREQUAL "OpenBSD" AND NOT CMAKE_CROSSCOMPILING)
-    include_directories(SYSTEM "/usr/local/include")
-  endif()
 endif()
 
 # Create a symlink for swift-api-dump.py in the bin directory