[libc++] Improves clang-tidy configuration.

[Mordante]

Summary

The current clang-tidy settings work in the CI but not on all systems
outside the CI.

• The range 16...17 doesn't work when only clang-17 is installed.
• Running CMake a second time will fail.

This addresses these issues.

[philnik]

Thanks! LGTM with green CI.

libcxx/test/tools/clang_tidy_checks/CMakeLists.txt

 
 # Look for the latest package
 set(CMAKE_FIND_PACKAGE_SORT_ORDER NATURAL)
 set(CMAKE_FIND_PACKAGE_SORT_DIRECTION DEC)
 
-find_package(Clang 16...17)
+# The find_package changes these variables. This leaves the build in an odd
+# state. Calling cmake a second time tries to write site config information in
+# the system's libc++. Restoring these setting after testing fixes this issue.
+set(LLVM_DIR_SAVE ${LLVM_DIR})
+set(Clang_DIR_SAVE ${Clang_DIR})
+
+find_package(Clang 16)
+if (NOT Clang_FOUND)
+  find_package(Clang 17)
+endif()
 
 set(SOURCES
     abi_tag_on_virtual.cpp
     hide_from_abi.cpp
     qualify_declval.cpp
     robust_against_adl.cpp
     uglify_attributes.cpp
 
     libcpp_module.cpp
    )
 
 if(NOT Clang_FOUND)
   message(STATUS "Could not find a suitable version of the Clang development package;
                   custom libc++ clang-tidy checks will not be available.")
   return()
 endif()
 
+set(LLVM_DIR "${LLVM_DIR_SAVE}" CACHE PATH "The directory containing a CMake configuration file for LLVM." FORCE)
+set(Clang_DIR "${Clang_DIR_SAVE}" CACHE PATH "The directory containing a CMake configuration file for Clang." FORCE)
+
 message(STATUS "Found system-installed LLVM ${LLVM_PACKAGE_VERSION} with headers in ${LLVM_INCLUDE_DIRS}")
 
 set(CMAKE_CXX_STANDARD 20)
 
 # Link only against clangTidy itself, not anything that clangTidy uses; otherwise we run setup code multiple times
 # which results in clang-tidy crashing
 set_target_properties(clangTidy PROPERTIES INTERFACE_LINK_LIBRARIES "")
 # ClangTargets.cmake doesn't set the include paths, so we have to do it