[lld-macho] Fix an assertion failure when -u specifies an undefined section$start symbol

This matches ld64. Also improve the test for `-dead_strip`.

Reviewed By: #lld-macho, Jez Ng

Differential Revision: https://reviews.llvm.org/D113147

lld/MachO/MarkLive.cpp

@@ -96,8 +96,7 @@ void markLive() {
   }
   // -u symbols
   for (Symbol *sym : config->explicitUndefineds)
-    if (auto *defined = dyn_cast<Defined>(sym))
-      addSym(defined);
+    addSym(sym);
   // local symbols explicitly marked .no_dead_strip
   for (const InputFile *file : inputFiles)
     if (auto *objFile = dyn_cast<ObjFile>(file))

lld/test/MachO/start-end.s

@@ -67,6 +67,13 @@
 
 ## Test that the link succeeds with dead-stripping enabled too.
 # RUN: %lld -dead_strip -lSystem %t/main.o -o %t/stripped.out
+# RUN: llvm-objdump --macho --syms --section-headers %t/stripped.out > %t-stripped-dump.txt
+# RUN: llvm-objdump --macho -d --no-symbolic-operands --no-show-raw-insn %t/stripped.out >> %t-stripped-dump.txt
+# RUN: FileCheck --check-prefix=STRIP %s < %t-stripped-dump.txt
+
+## -u 'section$start$*' does not cause an undefined symbol error. This matches ld64.
+# RUN: %lld -dead_strip -lSystem %t/main.o -u 'section$start$__FOO$__notexist' -o %t/stripped1.out
+# RUN: llvm-objdump --section-headers %t/stripped1.out | FileCheck --check-prefix=STRIP2 %s
 
 ## (Fun fact: `-e 'section$start$__TEXT$__text -dead_strip` strips
 ## everything in the text section because markLive runs well before
@@ -76,6 +83,42 @@
 ## and the output program crashes when running. This matches ld64's
 ## behavior.)
 
+# STRIP-LABEL: Sections:
+# STRIP-NEXT:  Idx Name           Size     VMA              Type
+# STRIP-NEXT:  0 __text           {{[0-9a-f]*}} [[#%x, TEXTSTART:]] TEXT
+# STRIP-NEXT:  1 __cstring        00000000      [[#%x, CSTRINGSTART:]] DATA
+# STRIP-NEXT:  2 __data           00000000
+# STRIP-NEXT:  3 __llvm_orderfile 00000000
+# STRIP-NEXT:  4 __mybss          00000000
+# STRIP-NEXT:  5 __bar            00000000
+# STRIP-NEXT:  6 __ever           00000000
+# STRIP-NEXT:  7 __lookup         00000000
+# STRIP-NEXT:  8 symbol           00000000
+# STRIP-NEXT:  9 __quux           00000000
+
+# STRIP-LABEL: SYMBOL TABLE:
+# STRIP-NOT:   section$start$__FOO$__bar
+
+# STRIP-LABEL: _main:
+# STRIP:      [[#%x, PC1:]]:
+# STRIP-SAME: leaq [[#%d, TEXTSTART - PC1 - 7]](%rip), %rax
+# STRIP-NEXT: [[#%x, PC2:]]:
+# STRIP-SAME: leaq [[#%d, CSTRINGSTART - PC2 - 7]](%rip), %rbx
+
+# STRIP2-LABEL: Sections:
+# STRIP2-NEXT:  Idx Name           Size     VMA              Type
+# STRIP2-NEXT:  0 __text           {{[0-9a-f]*}} [[#%x, TEXTSTART:]] TEXT
+# STRIP2-NEXT:  1 __cstring        00000000      [[#%x, CSTRINGSTART:]] DATA
+# STRIP2-NEXT:  2 __data           00000000
+# STRIP2-NEXT:  3 __llvm_orderfile 00000000
+# STRIP2-NEXT:  4 __mybss          00000000
+# STRIP2-NEXT:  5 __bar            00000000
+# STRIP2-NEXT:  6 __notexist       00000000
+# STRIP2-NEXT:  7 __ever           00000000
+# STRIP2-NEXT:  8 __lookup         00000000
+# STRIP2-NEXT:  9 symbol           00000000
+# STRIP2-NEXT:  10 __quux          00000000
+
 # CHECK-LABEL: Sections:
 # CHECK-NEXT:  Idx Name           Size     VMA              Type
 # CHECK:       0 __text           {{[0-9a-f]*}} [[#%x, TEXTSTART:]] TEXT