Make undefined symbol in DSO to pull out object files from archive files.

[ruiu]

Summary

We have an internal program that does't link without this patch. I don't
know of any open-source program that needs this, but there might be.
Since this patch improves compatibility with GNU linkers with a few lines
of code, I think it's worth to be committed.

The problem is about undefined symbols in DSOs. Some programs depend on
the GNU linkers' behavior that they pull out object files from archive
files to resolve undefined symbols in DSOs. We already allow that kind of
"reverse" dependency (from DSOs to the main executable) for regular
symbols, in particular, for "__progname" symbol, but that didn't work
if the symbol is in an archive file. This patch is to make it work.

[ruiu]

• updated the test case.

lld/ELF/SymbolTable.cpp

 // the definitions for them. An example is BSD's __progname symbol.
 // We need to put such symbols to the main program's .dynsym so that
 // shared libraries can find them.
 // Except this, we ignore undefined symbols in DSOs.
 template <class ELFT> void SymbolTable::scanShlibUndefined() {
   for (InputFile *F : SharedFiles) {
     for (StringRef U : cast<SharedFile<ELFT>>(F)->getUndefinedSymbols()) {
       Symbol *Sym = find(U);
-      if (!Sym || !Sym->isDefined())
+      if (!Sym)
         continue;
+      if (auto *L = dyn_cast<Lazy>(Sym))
+        if (InputFile *File = L->fetch())
+          addFile<ELFT>(File);
+
+      if (Sym->isDefined())
         Sym->ExportDynamic = true;
     }
   }
 }
 
 // Initialize DemangledSyms with a map from demangled symbols to symbol
 // objects. Used to handle "extern C++" directive in version scripts.
 //
 // The map will contain all demangled symbols. That can be very large,

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

+# REQUIRES: x86
+
+# Undefined symbols in a DSO should pull out object files from archives
+# to resolve them.
+
+# RUN: echo '.globl foo' | llvm-mc -filetype=obj -triple=x86_64-linux-gnu -o %t1.o -
+# RUN: ld.lld -shared -o %t.so %t1.o
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-linux-gnu -o %t2.o %s
+# RUN: llvm-ar cru %t.a %t2.o
+# RUN: ld.lld -o %t.exe %t.so %t.a
+# RUN: llvm-nm -D %t.exe | FileCheck %s
+
+# CHECK: T foo
+
+.globl foo
+foo:
+  ret