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

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.

Differential Revision: https://reviews.llvm.org/D112643

Author: nico

lld/MachO/ICF.cpp

@@ -246,7 +246,7 @@ void ICF::run() {
             } else {
               hash += defined->value;
             }
-          } else
+          } else if (!isa<Undefined>(sym))
             llvm_unreachable("foldIdenticalSections symbol kind");
         }
       }

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

@@ -3,7 +3,7 @@
 # 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)'