[Flang][experimental] Hide the -ffast-amd-memory-allocator in regualr help screen (llvm#2202)

Author: ronlieb

clang/include/clang/Driver/Options.td

@@ -3637,8 +3637,8 @@ defm disable_block_signature_string : BoolFOption<"disable-block-signature-strin
   BothFlags<[], [CC1Option], " block signature string)">>;
 
 def ffast_amd_memory_allocator : Flag<["-"], "ffast-amd-memory-allocator">, Group<f_Group>,
-  Visibility<[FlangOption,FC1Option]>,
-  HelpText<"Enable optimized memory allocator for AMD Instinct(tm) APUs">;
+  Flags<[HelpHidden]>, Visibility<[FlangOption,FC1Option]>,
+  HelpText<"Enable optimized memory allocator for AMD Instinct(tm) APUs (experimental)">;
 def fomit_frame_pointer : Flag<["-"], "fomit-frame-pointer">, Group<f_Group>,
   Visibility<[ClangOption, FlangOption]>,
   HelpText<"Omit the frame pointer from functions that don't need it. "