[libc] Add a linting target named "libc-lint".

[sivachandra]

Summary

Lint targets for individual entrypoints have also been cleaned up. The
target "libc-lint" depends on the individual lint targets.

libc/CMakeLists.txt

if(LLVM_LIBC_CLANG_TIDY)
         message(WARNING "
           'clang-tidy' (version ${CLANG_TIDY_VERSION}) is not the same as
           'clang' (version ${CLANG_MAJOR_VERSION}).  Linting will
           be disabled.
 
           The path to the clang-tidy binary can be set manually by passing
           -DLLVM_LIBC_CLANG_TIDY=<path/to/clang-tidy> to CMake.")
       endif()
+      add_custom_target(libc-lint)
     else()
       message(FATAL_ERROR "
         Linting is enabled but 'clang-tidy' is not found!
 
         The path to the clang-tidy binary can be set manually by passing
         -DLLVM_LIBC_CLANG_TIDY=<path/to/clang-tidy> to CMake.
 
         To disable linting set LLVM_LIBC_ENABLE_LINTING to OFF

libc/cmake/modules/LLVMLibCObjectRules.cmake

if(COMPILER_RESOURCE_DIR)
         -p ${PROJECT_BINARY_DIR}
         ${ADD_ENTRYPOINT_OBJ_SRCS}
       )
     else()
       set(restrict_system_headers_check_invocation
         COMMAND ${CMAKE_COMMAND} -E echo "Header file check skipped")
     endif()
 
-    set(lint_timestamp "${CMAKE_CURRENT_BINARY_DIR}/.${target_name}.__lint_timestamp__")
-    add_custom_command(
-      OUTPUT ${lint_timestamp}
+    add_custom_target(
+      ${fq_target_name}.__lint__
       # --quiet is used to surpress warning statistics from clang-tidy like:
       #     Suppressed X warnings (X in non-user code).
       # There seems to be a bug in clang-tidy where by even with --quiet some
       # messages from clang's own diagnostics engine leak through:
       #     X warnings generated.
       # Until this is fixed upstream, we use -fno-caret-diagnostics to surpress
       # these.
       COMMAND ${LLVM_LIBC_CLANG_TIDY}
               "--extra-arg=-fno-caret-diagnostics" --quiet
               # Path to directory containing compile_commands.json
               -p ${PROJECT_BINARY_DIR}
               ${ADD_ENTRYPOINT_OBJ_SRCS}
       # See above: this might be a second invocation of clang-tidy depending on
       # the conditions above.
       ${restrict_system_headers_check_invocation}
       # We have two options for running commands, add_custom_command and
       # add_custom_target. We don't want to run the linter unless source files
       # have changed. add_custom_target explicitly runs everytime therefore we
       # use add_custom_command. This function requires an output file and since
       # linting doesn't produce a file, we create a dummy file using a
       # crossplatform touch.
-      COMMAND "${CMAKE_COMMAND}" -E touch ${lint_timestamp}
-      COMMENT "Linting... ${target_name}"
-      DEPENDS clang-tidy ${internal_target_name} ${ADD_ENTRYPOINT_OBJ_SRCS}
+      COMMENT "Linting... ${fq_target_name}"
+      DEPENDS ${internal_target_name} ${ADD_ENTRYPOINT_OBJ_SRCS}
       WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
     )
+    add_dependencies(libc-lint ${fq_target_name}.__lint__)
   endif()
 
 endfunction(create_entrypoint_object)
 
 # Internal function, used by `add_entrypoint_object`.
 function(expand_flags_for_entrypoint_object target_name flags)
   cmake_parse_arguments(
     "EXPAND_FLAGS"

libc/test/src/CMakeLists.txt

add_integration_test(
   DEPENDS
     ${api-test-entrypoints}
 )
 
 if(COMPILER_RESOURCE_DIR AND LLVM_LIBC_ENABLE_LINTING)
   add_custom_target(
     libc-api-test-tidy
     VERBATIM
-    COMMAND $<TARGET_FILE:clang-tidy> --system-headers
+    COMMAND ${LLVM_LIBC_CLANG_TIDY} --system-headers
       --checks=-*,llvmlibc-restrict-system-libc-headers
       "--extra-arg=-resource-dir=${COMPILER_RESOURCE_DIR}"
       --header-filter=.*
       --warnings-as-errors=llvmlibc-*
       "-config={CheckOptions: [{key: llvmlibc-restrict-system-libc-headers.Includes, value: '-*, linux/*, asm/*.h, asm-generic/*.h'}]}"
       --quiet
       -p ${PROJECT_BINARY_DIR}
       ${public_test}
     DEPENDS
       clang-tidy ${public_test}
   )
   add_dependencies(libc-api-test libc-api-test-tidy)
 endif()