[cmake] switch to CMake's native check_{compiler,linker}_flag (#96171)

Broken out from #93429

Somewhat closing the loop opened by 7017e6c9cf.

Co-authored-by: Ryan Prichard <rprichard@google.com>

cmake/Modules/LLVMCheckCompilerLinkerFlag.cmake

@@ -1,14 +1,8 @@
 include(CMakePushCheckState)
-
-include(CheckCompilerFlag OPTIONAL)
-
-if(NOT COMMAND check_compiler_flag)
-  include(CheckCCompilerFlag)
-  include(CheckCXXCompilerFlag)
-endif()
+include(CheckCompilerFlag)
 
 function(llvm_check_compiler_linker_flag lang flag out_var)
-  # If testing a flag with check_c_compiler_flag, it gets added to the compile
+  # If testing a flag with check_compiler_flag, it gets added to the compile
   # command only, but not to the linker command in that test. If the flag
   # is vital for linking to succeed, the test would fail even if it would
   # have succeeded if it was included on both commands.
@@ -18,18 +12,6 @@ function(llvm_check_compiler_linker_flag lang flag out_var)
 
   cmake_push_check_state()
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} ${flag}")
-  if(COMMAND check_compiler_flag)
-    check_compiler_flag("${lang}" "" ${out_var})
-  else()
-    # Until the minimum CMAKE version is 3.19
-    # cmake builtin compatible, except we assume lang is C or CXX
-    if("${lang}" STREQUAL "C")
-      check_c_compiler_flag("" ${out_var})
-    elseif("${lang}" STREQUAL "CXX")
-      check_cxx_compiler_flag("" ${out_var})
-    else()
-      message(FATAL_ERROR "\"${lang}\" is not C or CXX")
-    endif()
-  endif()
+  check_compiler_flag("${lang}" "" ${out_var})
   cmake_pop_check_state()
 endfunction()