[lld-macho] Downgrade "cannot export hidden symbol" to warning

[int3]

Summary

This matches ld64's behavior, and makes it easier to fit LLD
into existing build systems.

[smeenai]

LGTM. Turning on fatal linker warnings would make this an error anyway, so this makes sense to me.

lld/MachO/Driver.cpp

if (config->timeTraceEnabled)
     createSyntheticSymbols();
 
     if (!config->exportedSymbols.empty()) {
       for (Symbol *sym : symtab->getSymbols()) {
         if (auto *defined = dyn_cast<Defined>(sym)) {
           StringRef symbolName = defined->getName();
           if (config->exportedSymbols.match(symbolName)) {
             if (defined->privateExtern) {
-              error("cannot export hidden symbol " + symbolName +
-                    "\n>>> defined in " + toString(defined->getFile()));
+              warn("cannot export hidden symbol " + symbolName +
             }
           } else {
             defined->privateExtern = true;
           }
         }
       }
     } else if (!config->unexportedSymbols.empty()) {
       for (Symbol *sym : symtab->getSymbols())

lld/test/MachO/export-options.s

 # RUN:         -exported_symbol _keep_lazy
 # RUN: llvm-objdump --syms %t/lazydef | \
 # RUN:     FileCheck --check-prefix=EXPORT %s
 
 # EXPORT-DAG: g     F __TEXT,__text _keep_globl
 # EXPORT-DAG: g     F __TEXT,__text _keep_lazy
 
 ## Check that exported symbol is global
-# RUN: not %lld -dylib %t/default.o -o /dev/null \
+# RUN: %no_fatal_warnings_lld -dylib %t/default.o -o %t/hidden-export \
 # RUN:         -exported_symbol _private_extern 2>&1 | \
 # RUN:     FileCheck --check-prefix=PRIVATE %s
 
-# PRIVATE: error: cannot export hidden symbol _private_extern
+# PRIVATE: warning: cannot export hidden symbol _private_extern
+
+## Check that we still hide the other symbols despite the warning
+# RUN: llvm-objdump --macho --exports-trie %t/hidden-export | \
+# RUN:     FileCheck --check-prefix=EMPTY-TRIE %s
+# EMPTY-TRIE:       Exports trie:
+# EMPTY-TRIE-EMPTY:
 
 ## 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