[libc++] Re-enable Clang-tidy checks in the CI

Author: ldionne

libcxx/test/tools/clang_tidy_checks/CMakeLists.txt

@@ -1,6 +1,3 @@
-# TODO: Re-enable the tests once the CI is back under control
-return()
-
 # 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.