Add a doc for missing key function and an error message referencing the doc.

[ruiu]

Summary

This is a common error, and because many people don't know what the key
function is, it is sometimes very confusing.

The doc was originally written by Brooks Moses and slightly edited by me.

[MaskRay]

As a reference, gold added this in https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=93acabad6a0ff499e4e18d44c41292ef48cde104

This is an extremely common user error and cause confusion so we ought to add it.

[ruiu]

• add a document about missing key function error

[echristo]

lld/docs/missingkeymethod.rst

82 ↗

(On Diff #179312)

Perhaps change "build file" to "dependencies"?

[ruiu]

• review comments

[echristo]

LGTM.

lld/ELF/Relocations.cpp

static bool maybeReportUndefined(Symbol &Sym, InputSectionBase &Sec,
   std::string Msg =
       "undefined symbol: " + toString(Sym) + "\n>>> referenced by ";
 
   std::string Src = Sec.getSrcMsg(Sym, Offset);
   if (!Src.empty())
     Msg += Src + "\n>>>               ";
   Msg += Sec.getObjMsg(Offset);
 
+  if (Sym.getName().startswith("_ZTV"))
+    Msg += "\nthe vtable symbol may be undefined because the class is "
+           "missing its key function";
+
   if ((Config->UnresolvedSymbols == UnresolvedPolicy::Warn && CanBeExternal) ||
       Config->NoinhibitExec) {
     warn(Msg);
     return false;
   }
 
   error(Msg);
   return true;

lld/test/ELF/undef.s

 # CHECK: error: undefined symbol: bar
 # CHECK: >>> referenced by undef.s
 # CHECK: >>>               {{.*}}:(.text+0x6)
 
 # CHECK: error: undefined symbol: foo(int)
 # CHECK: >>> referenced by undef.s
 # CHECK: >>>               {{.*}}:(.text+0x10)
 
+# CHECK: error: undefined symbol: vtable for Foo
+# CHECK: the vtable symbol may be undefined because the class is missing its key function
+
 # CHECK: error: undefined symbol: zed2
 # CHECK: >>> referenced by {{.*}}.o:(.text+0x0) in archive {{.*}}2.a
 
 # CHECK: error: undefined symbol: zed3
 # CHECK: >>> referenced by undef-debug.s:3 (dir{{/|\\}}undef-debug.s:3)
 # CHECK: >>>               {{.*}}.o:(.text+0x0)
 
 # CHECK: error: undefined symbol: zed4

 .file "undef.s"
 
   .globl _start
 _start:
   call foo
   call bar
   call zed1
   call _Z3fooi
+  call _ZTV3Foo