[lld] Rename -flavor darwin to -flavor old-darwin

[MaskRay]

Summary

@lhames agreed to remove the existing Mach-O port from lld
(https://reviews.llvm.org/D75382#1906108), but we should probably
continue keeping it until the new Mach-O port gets matured. The existing
Mach-O is also used by some downstream projects like Zig.

This patch is similar to rL253188 "Rename the gnu flavour to gnu old."
When the new Mach-O port gets matured, we can make a change similar to rL262158: "Remove the old ELF linker."

[ruiu]

But we are going to call the new one as mach_o2 until mach_o can be removed and rename the new one mach_o once it's done, no? If so, I don't think we need to rename only this part at this moment. We can rename them all at once later.

[smeenai]

Sorry, this slipped through the cracks, but I agree with @ruiu .

lld/test/mach-o/usage.yaml

 # RUN: not ld64.lld | FileCheck %s
+# RUN: not lld -flavor old-darwin | FileCheck %s
 #
 # Test that running darwin linker with no option prints out usage message.
 #
 
 
 # CHECK:            USAGE:
 # CHECK:            -arch

lld/tools/lld/lld.cpp

 #include <cstdlib>
 
 using namespace lld;
 using namespace llvm;
 using namespace llvm::sys;
 
 enum Flavor {
   Invalid,
-  Gnu,     // -flavor gnu
-  WinLink, // -flavor link
-  Darwin,  // -flavor darwin
-  Wasm,    // -flavor wasm
+  Gnu,       // -flavor gnu
+  WinLink,   // -flavor link
+  OldDarwin, // -flavor old-darwin
+  Wasm,      // -flavor wasm
 };
 
 LLVM_ATTRIBUTE_NORETURN static void die(const Twine &s) {
   llvm::errs() << s << "\n";
   exit(1);
 }
 
 static Flavor getFlavor(StringRef s) {
   return StringSwitch<Flavor>(s)
       .CasesLower("ld", "ld.lld", "gnu", Gnu)
       .CasesLower("wasm", "ld-wasm", Wasm)
       .CaseLower("link", WinLink)
-      .CasesLower("ld64", "ld64.lld", "darwin", Darwin)
+      .CasesLower("ld64", "ld64.lld", "old-darwin", OldDarwin)
       .Default(Invalid);
 }
 
 static cl::TokenizerCallback getDefaultQuotingStyle() {
   if (Triple(sys::getProcessTriple()).getOS() == Triple::Win32)
     return cl::TokenizeWindowsCommandLine;
   return cl::TokenizeGNUCommandLine;
 }
int main(int argc, const char **argv) {
   std::vector<const char *> args(argv, argv + argc);
   switch (parseFlavor(args)) {
   case Gnu:
     if (isPETarget(args))
       return !mingw::link(args, canExitEarly(), llvm::outs(), llvm::errs());
     return !elf::link(args, canExitEarly(), llvm::outs(), llvm::errs());
   case WinLink:
     return !coff::link(args, canExitEarly(), llvm::outs(), llvm::errs());
-  case Darwin:
+  case OldDarwin:
     return !mach_o::link(args, canExitEarly(), llvm::outs(), llvm::errs());
   case Wasm:
     return !wasm::link(args, canExitEarly(), llvm::outs(), llvm::errs());
   default:
     die("lld is a generic driver.\n"
         "Invoke ld.lld (Unix), ld64.lld (macOS), lld-link (Windows), wasm-ld"
         " (WebAssembly) instead");
   }
 }