[lld/mac] warn on -install_name without -dylib

[thakis]

Summary

The flag doesn't (and shouldn't) have an effect in that case.
ld64 doesn't warn on this, but it seems like a good thing to do.
If it causes problems in practice for some reason, we can revert it.

Also add a dedicated test for install_name.

[int3]

lgtm, though I wonder if we can make some sort of tblgen macro to make specifying these behaviors more terse

lld/MachO/Driver.cpp

for (auto *arg : args.filtered(OPT_u)) {
     config->explicitUndefineds.push_back(symtab->addUndefined(
         arg->getValue(), /*file=*/nullptr, /*isWeakRef=*/false));
   }
 
   for (auto *arg : args.filtered(OPT_U))
     symtab->addDynamicLookup(arg->getValue());
 
   config->outputFile = args.getLastArgValue(OPT_o, "a.out");
-  config->installName =
-      args.getLastArgValue(OPT_install_name, config->outputFile);
+
   config->headerPad = args::getHex(args, OPT_headerpad, /*Default=*/32);
   config->headerPadMaxInstallNames =
       args.hasArg(OPT_headerpad_max_install_names);
   config->printEachFile = args.hasArg(OPT_t);
   config->printWhyLoad = args.hasArg(OPT_why_load);
   config->outputType = getOutputType(args);
   if (const opt::Arg *arg = args.getLastArg(OPT_bundle_loader)) {
     if (config->outputType != MH_BUNDLE)
       error("-bundle_loader can only be used with MachO bundle output");
     addFile(arg->getValue(), false, true);
   }
   config->ltoObjPath = args.getLastArgValue(OPT_object_path_lto);
   config->ltoNewPassManager =
       args.hasFlag(OPT_no_lto_legacy_pass_manager, OPT_lto_legacy_pass_manager,
                    LLVM_ENABLE_NEW_PASS_MANAGER);
   config->runtimePaths = args::getStrings(args, OPT_rpath);
   config->allLoad = args.hasArg(OPT_all_load);
   config->forceLoadObjC = args.hasArg(OPT_ObjC);
   config->demangle = args.hasArg(OPT_demangle);
   config->implicitDylibs = !args.hasArg(OPT_no_implicit_dylibs);
 
+  if (const opt::Arg *arg = args.getLastArg(OPT_install_name)) {
+    if (config->outputType != MH_DYLIB)
+      warn(arg->getAsString(args) + ": ignored, only has effect with -dylib");
+    else
+      config->installName = arg->getValue();
+  } else if (config->outputType == MH_DYLIB) {
+    config->installName = config->outputFile;
+  }
+
   if (const opt::Arg *arg = args.getLastArg(OPT_static, OPT_dynamic))
     config->staticLink = (arg->getOption().getID() == OPT_static);
 
   if (const opt::Arg *arg =
           args.getLastArg(OPT_flat_namespace, OPT_twolevel_namespace))
     config->namespaceKind = arg->getOption().getID() == OPT_twolevel_namespace
                                 ? NamespaceKind::twolevel
                                 : NamespaceKind::flat;

lld/test/MachO/install-name.s

+# REQUIRES: x86
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos -o %t.o %s
+
+# RUN: %no_fatal_warnings_lld -o %t.exec %t.o -install_name foo 2>&1 \
+# RUN:     | FileCheck --check-prefix=WARN %s
+# RUN: llvm-objdump --macho --all-headers %t.exec \
+# RUN:     | FileCheck --check-prefix=NO-ID %s
+
+# RUN: %no_fatal_warnings_lld -bundle -o %t.bundle %t.o -install_name foo 2>&1 \
+# RUN:     | FileCheck --check-prefix=WARN %s
+# RUN: llvm-objdump --macho --all-headers %t.bundle \
+# RUN:     | FileCheck --check-prefix=NO-ID %s
+
+# RUN: %lld -dylib -o %t.dylib %t.o -install_name foo 2>&1
+# RUN: llvm-objdump --macho --all-headers %t.dylib \
+# RUN:     | FileCheck --check-prefix=ID %s
+
+# WARN: warning: -install_name foo: ignored, only has effect with -dylib
+
+# NO-ID-NOT: LC_ID_DYLIB
+
+# ID:          cmd LC_ID_DYLIB
+# ID-NEXT: cmdsize
+# LID-NEXT:   name foo
+
+
+.globl _main
+_main:
+  ret