Add -print-targets to print the registered targets

[yaxunl]

add option -print-targets to print the registered targets.

[MaskRay]

Thanks!

clang/docs/ReleaseNotes.rst

 - The deprecated argument ``-f[no-]sanitize-recover`` has changed to mean
   ``-f[no-]sanitize-recover=all`` instead of
   ``-f[no-]sanitize-recover=undefined,integer`` and is no longer deprecated.
 - The argument to ``-f[no-]sanitize-trap=...`` is now optional and defaults to
   ``all``.
 - ``-fno-char8_t`` now disables the ``char8_t`` keyword, not just the use of
   ``char8_t`` as the character type of ``u8`` literals. This restores the
   Clang 8 behavior that regressed in Clang 9 and 10.
+- -print-targets has been added to print the registered targets.
 
 New Pragmas in Clang
 --------------------
 
 - ...
 
 Attribute Changes in Clang
 --------------------------

clang/include/clang/Driver/Options.td

 def print_effective_triple : Flag<["-", "--"], "print-effective-triple">,
   HelpText<"Print the effective target triple">;
 def print_prog_name_EQ : Joined<["-", "--"], "print-prog-name=">,
   HelpText<"Print the full program path of <name>">, MetaVarName<"<name>">;
 def print_resource_dir : Flag<["-", "--"], "print-resource-dir">,
   HelpText<"Print the resource directory pathname">;
 def print_search_dirs : Flag<["-", "--"], "print-search-dirs">,
   HelpText<"Print the paths used for finding libraries and programs">;
+def print_targets : Flag<["-", "--"], "print-targets">,
+  HelpText<"Print the registered targets">;
 def private__bundle : Flag<["-"], "private_bundle">;
 def pthreads : Flag<["-"], "pthreads">;
 def pthread : Flag<["-"], "pthread">, Flags<[CC1Option]>,
   HelpText<"Support POSIX threads in generated code">;
 def no_pthread : Flag<["-"], "no-pthread">, Flags<[CC1Option]>;
 def p : Flag<["-"], "p">;
 def pie : Flag<["-"], "pie">;
 def static_pie : Flag<["-"], "static-pie">;

clang/lib/Driver/Driver.cpp

bool Driver::HandleImmediateArgs(const Compilation &C) {
   }
 
   if (C.getArgs().hasArg(options::OPT_print_effective_triple)) {
     const llvm::Triple Triple(TC.ComputeEffectiveClangTriple(C.getArgs()));
     llvm::outs() << Triple.getTriple() << "\n";
     return false;
   }
 
+  if (C.getArgs().hasArg(options::OPT_print_targets)) {
+    llvm::TargetRegistry::printRegisteredTargetsForVersion(llvm::outs());
+    return false;
+  }
+
   return true;
 }
 
 enum {
   TopLevelAction = 0,
   HeadSibAction = 1,
   OtherSibAction = 2,
 };