[lld-macho] Downgrade missing fat arch to warning

[keith]

Summary

This mirrors ld64's behavior. In many cases this likely still leads to a
link failure but if you didn't actually use anything from the library
it can be ignored. If you care about these invalid cases -fatal_warnings
still upgrades it back to an error.

Example: https://github.com/keith/ld64.lld/issues/3

[int3]

Might be nice for us to add more detail in the error/warning message indicating the arch looked for vs the archs actually found. Also the message should be standardized into the $path: $message form. We can do that later though, this diff is fine as-is

lld/MachO/InputFiles.cpp

for (uint32_t i = 0, n = read32be(&hdr->nfat_arch); i < n; ++i) {
     if (offset + size > mbref.getBufferSize())
       error(path + ": slice extends beyond end of file");
     if (tar)
       tar->append(relativeToRoot(path), mbref.getBuffer());
     return cachedReads[key] = MemoryBufferRef(StringRef(buf + offset, size),
                                               path.copy(bAlloc));
   }
 
-  error("unable to find matching architecture in " + path);
+  warn("unable to find matching architecture in " + path);
   return std::nullopt;
 }
 
 InputFile::InputFile(Kind kind, const InterfaceFile &interface)
     : id(idCount++), fileKind(kind), name(saver().save(interface.getPath())) {}
 
 // Some sections comprise of fixed-size records, so instead of splitting them at
 // symbol boundaries, we split them based on size. Records are distinct from

lld/test/MachO/fat-arch.s

 # REQUIRES: x86,aarch64
 ## FIXME: The tests doesn't run on windows right now because of llvm-mc (can't produce triple=arm64-apple-macos11.0)
 # UNSUPPORTED: system-windows
 
 # RUN: llvm-mc -filetype=obj -triple=i386-apple-darwin %s -o %t.i386.o
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %s -o %t.x86_64.o
 # RUN: llvm-mc -filetype=obj -triple=arm64-apple-macos11.0 %s -o %t.arm64.o
 
 # RUN: llvm-lipo %t.i386.o %t.x86_64.o -create -o %t.fat.o
 # RUN: %lld -o /dev/null %t.fat.o
 # RUN: llvm-lipo %t.i386.o -create -o %t.noarch.o
-# RUN: not %lld -o /dev/null %t.noarch.o 2>&1 | \
+# RUN: not %no-fatal-warnings-lld -o /dev/null %t.noarch.o 2>&1 | \
 # RUN:    FileCheck %s -DFILE=%t.noarch.o
-# CHECK: error: unable to find matching architecture in [[FILE]]
+# CHECK: warning: unable to find matching architecture in [[FILE]]
 
 ## Validates that we read the cpu-subtype correctly from a fat exec.
 # RUN: %lld -o %t.x86_64.out %t.x86_64.o
 # RUN: %lld -arch arm64 -o %t.arm64.out %t.arm64.o
 # RUN: llvm-lipo %t.x86_64.out %t.arm64.out -create -o %t.fat.exec.out
 # RUN: %lld -arch x86_64 %t.x86_64.o -bundle_loader %t.fat.exec.out -bundle -o %t.fat.bundle
 
 # RUN: llvm-otool -h %t.fat.bundle > %t.bundle_header.txt
 # RUN: llvm-otool -f %t.fat.exec.out >> %t.bundle_header.txt
 # RUN: cat %t.bundle_header.txt | FileCheck %s --check-prefix=CPU-SUB
 
 # CPU-SUB:            magic     cputype      cpusubtype   caps      filetype   ncmds sizeofcmds      flags
 # CPU-SUB-NEXT:  0xfeedfacf     16777223              3  0x{{.+}}    {{.+}}  {{.+}}    {{.+}}      {{.+}}
-  
 # CPU-SUB: Fat headers
 # CPU-SUB: nfat_arch 2
 # CPU-SUB: architecture 0
 # CPU-SUB-NEXT:    cputype 16777223
 # CPU-SUB-NEXT:    cpusubtype 3
 # CPU-SUB: architecture 1
 # CPU-SUB-NEXT:    cputype 16777228
 # CPU-SUB-NEXT:    cpusubtype 0
 
 .text
 .global _main
 _main:
   ret