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

--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).

Reviewed By: grimar, psmith

Differential Revision: https://reviews.llvm.org/D81028

lld/ELF/Writer.cpp

@@ -1980,7 +1980,8 @@ template <class ELFT> void Writer<ELFT>::finalizeSections() {
       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-

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

@@ -9,6 +9,7 @@
 # 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
 
@@ -27,4 +28,4 @@
 _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

@@ -6,7 +6,7 @@
 
 ## --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: {{.*}}.so: 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