[lld/mac] Don't crash on undefined symbols with --icf=all

[thakis]

Summary

ICF runs before relocation processing, but undefined symbol errors
are only emitted during relocation processing.

So just ignore Undefineds during ICF (instead of crashing) -- lld
will emit an error once ICF is done.

Fixes PR52330.

[int3]

Thanks!

[int3]

I guess this was because ICF didn't use to run before reloc processing

lld/MachO/ICF.cpp

parallelForEach(icfInputs, [&](ConcatInputSection *isec) {
               if (auto isec = dyn_cast<ConcatInputSection>(defined->isec))
                 hash += defined->value + isec->icfEqClass[icfPass % 2];
               else
                 hash += defined->isec->kind() +
                         defined->isec->getOffset(defined->value);
             } else {
               hash += defined->value;
             }
-          } else
+          } else if (!isa<Undefined>(sym))
             llvm_unreachable("foldIdenticalSections symbol kind");
         }
       }
       // Set MSB to 1 to avoid collisions with non-hashed classes.
       isec->icfEqClass[(icfPass + 1) % 2] = hash | (1ull << 63);
     });
   }
 

lld/test/MachO/invalid/undefined-symbol.s

 # REQUIRES: x86
 # RUN: rm -rf %t; split-file %s %t
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/main.s -o %t/main.o
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/foo.s -o %t/foo.o
 # RUN: llvm-ar crs %t/foo.a %t/foo.o
-# RUN: not %lld -o /dev/null %t/main.o 2>&1 | \
+# RUN: not %lld --icf=all -o /dev/null %t/main.o 2>&1 | \
 # RUN:     FileCheck %s -DSYM=_foo -DFILENAME=%t/main.o
 # RUN: not %lld -o /dev/null %t/main.o %t/foo.a 2>&1 | \
 # RUN:     FileCheck %s -DSYM=_bar -DFILENAME='%t/foo.a(foo.o)'
 # RUN: not %lld -o /dev/null %t/main.o -force_load %t/foo.a 2>&1 | \
 # RUN:     FileCheck %s -DSYM=_bar -DFILENAME='%t/foo.a(foo.o)'
 # CHECK: error: undefined symbol: [[SYM]]
 # CHECK-NEXT: >>> referenced by [[FILENAME]]