OpenMP/cmake: Use DEPFILE instead of IMPLICIT_DEPENDS

[arsenm]

Summary

IMPLICIT_DEPENDS doesn't actually work with ninja and this does.

[jhuber6]

I want to gut all of this code at some point, but that's for fixing this.

[arsenm]

a2f5bcc766fb77118630e13c91c3f3f2fd43420a

openmp/libomptarget/DeviceRTL/CMakeLists.txt

 function(compileDeviceRTLLibrary target_cpu target_name target_triple)
   set(target_bc_flags ${ARGN})
 
   set(bc_files "")
   foreach(src ${src_files})
     get_filename_component(infile ${src} ABSOLUTE)
     get_filename_component(outfile ${src} NAME)
     set(outfile "${outfile}-${target_cpu}.bc")
+    set(depfile "${outfile}.d")
 
     add_custom_command(OUTPUT ${outfile}
       COMMAND ${CLANG_TOOL}
       ${bc_flags}
       --offload-arch=${target_cpu}
       ${target_bc_flags}
+      -MD -MF ${depfile}
       ${infile} -o ${outfile}
-      DEPENDS ${infile} ${include_files}
-      IMPLICIT_DEPENDS CXX ${infile}
+      DEPENDS ${infile}
+      DEPFILE ${depfile}
       COMMENT "Building LLVM bitcode ${outfile}"
       VERBATIM
     )
     if("${CLANG_TOOL}" STREQUAL "$<TARGET_FILE:clang>")
       # Add a file-level dependency to ensure that clang is up-to-date.
       # By default, add_custom_command only builds clang if the
       # executable is missing.
       add_custom_command(OUTPUT ${outfile}