[libc++] Use clang-tidy version that matches the compiler we use in the CI (#85305)

This works around ODR violations in the clang-tidy plugin we use to
perform the modules tests.

Fixes #85242

Author: ldionne

libcxx/test/tools/clang_tidy_checks/CMakeLists.txt

@@ -5,7 +5,7 @@
 set(LLVM_DIR_SAVE ${LLVM_DIR})
 set(Clang_DIR_SAVE ${Clang_DIR})
 
-find_package(Clang 18)
+find_package(Clang 18.1)
 if (NOT Clang_FOUND)
   find_package(Clang 17)
 endif()