[llvm-libtool-darwin] Improve warning message for no symbols

[keith]

Summary

This more closely mirrors apple's libtool, and also potentially makes it
clearer for multi-arch archives where the issue lies.

[keith]

Update formatting

[keith]

Fix word

[smeenai]

LGTM

llvm/test/tools/llvm-libtool-darwin/no-symbols-warning.test

 ## This test verifies that the tool emits a warning for object files
 ## without symbols.
 
 # RUN: yaml2obj --docnum=1 %s -o %t-x86_64-empty.o
 # RUN: yaml2obj --docnum=2 %s -o %t-armv7-empty.o
 # RUN: echo 'target triple = "arm64-apple-ios8.0.0"' | llvm-as -o %t-empty.bc
 # RUN: yaml2obj %S/Inputs/input1.yaml -o %t-non-empty.o
 
 # RUN: llvm-libtool-darwin -static -o %t.lib %t-x86_64-empty.o 2>&1 | \
 # RUN:   FileCheck --check-prefix=WARNING %s -DPREFIX=%basename_t.tmp
 
-# WARNING: warning: [[PREFIX]]-x86_64-empty.o has no symbols
+# WARNING: warning: '[[PREFIX]]-x86_64-empty.o': has no symbols for architecture x86_64
 
 # RUN: llvm-libtool-darwin -no_warning_for_no_symbols -static -o %t.lib \
 # RUN:   %t-x86_64-empty.o 2>&1 | \
 # RUN:   FileCheck %s --allow-empty --implicit-check-not='warning:'
 
 # RUN: llvm-libtool-darwin -static -o %t.lib %t-non-empty.o 2>&1 | \
 # RUN:   FileCheck %s --allow-empty --implicit-check-not='warning:'
 

llvm/tools/llvm-libtool-darwin/llvm-libtool-darwin.cpp

Error verifyAndAddMachOObject(NewArchiveMember Member) {
       // Throw error if not in Mach-O format.
       if (!isa<object::MachOObjectFile>(**ObjOrErr))
         return createStringError(std::errc::invalid_argument,
                                  "'%s': format not supported",
                                  Member.MemberName.data());
 
       auto *O = dyn_cast<MachOObjectFile>(ObjOrErr->get());
       uint32_t FileCPUType, FileCPUSubtype;
+      StringRef ArchName = O->getArchTriple().getArchName();
       std::tie(FileCPUType, FileCPUSubtype) = MachO::getCPUTypeFromArchitecture(
-          MachO::getArchitectureFromName(O->getArchTriple().getArchName()));
+          MachO::getArchitectureFromName(ArchName));
 
       // If -arch_only is specified then skip this file if it doesn't match
       // the architecture specified.
       if (!ArchType.empty() && !acceptFileArch(FileCPUType, FileCPUSubtype)) {
         return Error::success();
       }
 
       if (!NoWarningForNoSymbols && O->symbols().empty())
-        WithColor::warning() << Member.MemberName + " has no symbols\n";
+        WithColor::warning() << "'" + Member.MemberName +
+                                    "': has no symbols for architecture " +
+                                    ArchName + "\n";
 
       uint64_t FileCPUID = getCPUID(FileCPUType, FileCPUSubtype);
       Builder.Data.MembersPerArchitecture[FileCPUID].push_back(
           std::move(Member), FileName);
       return Error::success();
     }
 
     Error verifyAndAddIRObject(NewArchiveMember Member) {