[ELF] Append " [--no-allow-shlib-undefined]" to the corresponding diagnostics

[MaskRay]

Summary

--no-allow-shlib-undefined (enabled by default when linking an
executable) rejects unresolved references in shared objects.

Users may be confused by the common diagnostics of unresolved symbols in
object files (LLD: "undefined symbol: foo"; GNU ld/gold: "undefined reference to")

Learn from GCC/clang " [-Wfoo]": append the option name to the
diagnostics. Users can find relevant information by searching
"--no-allow-shlib-undefined". It should also be obvious to them that
the positive form --allow-shlib-undefined can suppress the error.

Also downgrade the error to a warning if --noinhibit-exec is used (compatible
with GNU ld and gold).

[psmith]

LGTM, looks like a good idea. We may find other places where this form will be useful.

[grimar]

LGTM too.

[MaskRay]

Add a missing prefix in an error

lld/ELF/Writer.cpp

for (SharedFile *file : sharedFiles)
           llvm::all_of(file->dtNeeded, [&](StringRef needed) {
             return symtab->soNames.count(needed);
           });
 
     for (Symbol *sym : symtab->symbols())
       if (sym->isUndefined() && !sym->isWeak())
         if (auto *f = dyn_cast_or_null<SharedFile>(sym->file))
           if (f->allNeededIsKnown)
-            error(toString(f) + ": undefined reference to " + toString(*sym));
+            errorOrWarn(toString(f) + ": undefined reference to " +
+                        toString(*sym) + " [--no-allow-shlib-undefined]");
   }
 
   // Now that we have defined all possible global symbols including linker-
   // synthesized ones. Visit all symbols to give the finishing touches.
   for (Symbol *sym : symtab->symbols()) {
     if (!includeInSymtab(*sym))
       continue;
     if (in.symTab)

lld/test/ELF/allow-shlib-undefined.s

 # REQUIRES: x86
 
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux \
 # RUN:   %p/Inputs/allow-shlib-undefined.s -o %t1.o
 # RUN: ld.lld -shared %t1.o -o %t.so
 
 # RUN: ld.lld --allow-shlib-undefined %t.o %t.so -o /dev/null
 # RUN: not ld.lld --no-allow-shlib-undefined %t.o %t.so -o /dev/null 2>&1 | FileCheck %s
 # Executable defaults to --no-allow-shlib-undefined
 # RUN: not ld.lld %t.o %t.so -o /dev/null 2>&1 | FileCheck %s
+# RUN: ld.lld %t.o %t.so --noinhibit-exec -o /dev/null 2>&1 | FileCheck %s
 # -shared defaults to --allow-shlib-undefined
 # RUN: ld.lld -shared %t.o %t.so -o /dev/null
 
 # RUN: echo | llvm-mc -filetype=obj -triple=x86_64-unknown-linux -o %tempty.o
 # RUN: ld.lld -shared %tempty.o -o %tempty.so
 # RUN: ld.lld -shared %t1.o %tempty.so -o %t2.so
 # RUN: ld.lld --no-allow-shlib-undefined %t.o %t2.so -o /dev/null
 
 # DSO with undefines:
 # should link with or without any of these options.
 # RUN: ld.lld -shared %t1.o -o /dev/null
 # RUN: ld.lld -shared --allow-shlib-undefined %t1.o -o /dev/null
 # RUN: ld.lld -shared --no-allow-shlib-undefined %t1.o -o /dev/null
 
 .globl _start
 _start:
   callq _shared@PLT
 
-# CHECK: undefined reference to _unresolved
+# CHECK: {{.*}}.so: undefined reference to _unresolved [--no-allow-shlib-undefined]

lld/test/ELF/wrap-shlib-undefined.s

 # REQUIRES: x86
 
 # RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o
 # RUN: echo '.globl bar; bar: call __real_foo' | llvm-mc -filetype=obj -triple=x86_64 - -o %t1.o
 # RUN: ld.lld -shared -soname=t.so %t1.o -o %t.so
 
 ## --no-allow-shlib-undefined errors because __real_foo is not defined.
 # RUN: not ld.lld %t.o %t.so -o /dev/null 2>&1 | FileCheck --check-prefix=ERR %s
-# ERR: undefined reference to __real_foo
+# ERR: undefined reference to __real_foo [--no-allow-shlib-undefined]
 
 ## --wrap=foo defines __real_foo.
 # RUN: ld.lld %t.o %t.so --wrap=foo -o %t
 # RUN: llvm-readelf --dyn-syms %t | FileCheck %s
 
 ## FIXME GNU ld does not export __wrap_foo
 ## The reference __real_foo from %t.so causes foo to be exported.
 # CHECK:      Symbol table '.dynsym' contains 4 entries: