[lld][WebAssembly] Fix stub library deps causing LTO archive members to be required post-LTO

Fixes: emscripten-core/emscripten#16836
Fixes: emscripten-core/emscripten#20275

Author: sbc100

lld/test/wasm/lto/stub-library.s

@@ -1,12 +1,15 @@
 # RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown -o %t.o %s
-# RUN: llvm-as %S/Inputs/foo.ll -o %t1.o
-# RUN: wasm-ld %t.o %t1.o %p/Inputs/stub.so -o %t.wasm
+# RUN: mkdir -p %t
+# RUN: llvm-as %S/Inputs/foo.ll -o %t/foo.o
+# RUN: rm -f %t/libfoo.a
+# RUN: llvm-ar rcs %t/libfoo.a %t/foo.o
+# RUN: wasm-ld %t.o %t/libfoo.a %p/Inputs/stub.so -o %t.wasm
 # RUN: obj2yaml %t.wasm | FileCheck %s
 
-# The function `bar` is declared in stub.so and depends on `foo`, which happens
-# be in an LTO object.
-# This verifies that stub library dependencies (required exports) can be defined
-# in LTO objects.
+## The function `bar` is declared in stub.so and depends on `foo`, which happens
+## be in an LTO object, in an archive file.
+## This verifies that stub library dependencies (required exports) can be defined
+## in LTO objects.
 .functype bar () -> ()
 
 .globl _start

lld/wasm/Driver.cpp

@@ -949,6 +949,17 @@ static void processStubLibrariesPreLTO() {
           auto* needed = symtab->find(dep);
           if (needed ) {
             needed->isUsedInRegularObj = true;
+            // Like with handleLibcall we have extract any archive members
+            // that might need to be exported due to stub library symbol being
+            // used.  Without this the LTO object could be extracted during
+            // processStubLibraries, which is too late since LTO has already
+            // beeing performed at that point.
+            if (needed->isLazy() && isa<BitcodeFile>(needed->getFile())) {
+              if (!config->whyExtract.empty())
+                ctx.whyExtractRecords.emplace_back("<stubdep>",
+                                                   needed->getFile(), *needed);
+              cast<LazySymbol>(needed)->extract();
+            }
           }
         }
       }

lld/wasm/InputFiles.cpp

@@ -761,7 +761,7 @@ void StubFile::parse() {
     }
 
     // Lines starting with # are considered comments
-    if (line.starts_with("#"))
+    if (line.starts_with("#") || !line.size())
       continue;
 
     StringRef sym;