[lld/mac] Don't assert when using -exported_symbol with private symbol

[thakis]

Summary

When I added this assert in D93609, it asserted that a symbol that
is privateExtern is also isExternal().

In D98381 the privateExtern check moved into shouldExportSymbol()
but the assert didn't -- now it checked that _every_ non-exported
symbol is isExternal(), which isn't true. Move the assert into the
privateExtern check where it used to be.

Fixes PR50098.

[int3]

How about removing the assert altogether? InputFiles.cpp already has assert((sym.n_type & N_EXT) && "invalid input");

Keeping it is fine too I guess since it's a debug build only thing

lld/MachO/SyntheticSections.cpp

 static void validateExportSymbol(const Defined *defined) {
   StringRef symbolName = defined->getName();
   if (defined->privateExtern && config->exportedSymbols.match(symbolName))
     error("cannot export hidden symbol " + symbolName + "\n>>> defined in " +
           toString(defined->getFile()));
 }
 
 static bool shouldExportSymbol(const Defined *defined) {
-  if (defined->privateExtern)
+  if (defined->privateExtern) {
+    assert(defined->isExternal() && "invalid input file");
     return false;
+  }
   // TODO: Is this a performance bottleneck? If a build has mostly
   // global symbols in the input but uses -exported_symbols to filter
   // out most of them, then it would be better to set the value of
   // privateExtern at parse time instead of calling
   // exportedSymbols.match() more than once.
   //
   // Measurements show that symbol ordering (which again looks up
   // every symbol in a hashmap) is the biggest bottleneck when linking
for (const SymtabEntry &entry : concat<const SymtabEntry>(
            localSymbols, externalSymbols, undefinedSymbols)) {
     nList->n_strx = entry.strx;
     // TODO populate n_desc with more flags
     if (auto *defined = dyn_cast<Defined>(entry.sym)) {
       uint8_t scope = 0;
       if (!shouldExportSymbol(defined)) {
         // Private external -- dylib scoped symbol.
         // Promote to non-external at link time.
-        assert(defined->isExternal() && "invalid input file");
         scope = N_PEXT;
       } else if (defined->isExternal()) {
         // Normal global symbol.
         scope = N_EXT;
       } else {
         // TU-local symbol from localSymbols.
         scope = 0;
       }

lld/test/MachO/export-options.s

 .globl _keep_globl, _hide_globl
 _keep_globl:
   retq
 _hide_globl:
   retq
 .private_extern _private_extern
 _private_extern:
   retq
+_private:
+  retq
 
 ## Check that the export trie is unaltered
 # RUN: %lld -dylib %t/default.o -o %t/default
 # RUN: llvm-objdump --macho --exports-trie %t/default | \
 # RUN:     FileCheck --check-prefix=DEFAULT %s
 
 # DEFAULT-LABEL: Exports trie:
 # DEFAULT-DAG:   _hide_globl