[LLD][COFF] Allow symbols with empty chunks to have no associated output section

If a chunk is empty and there are no other non-empty chunks in the same section,
removeEmptySections() will remove the entire section. In this case, use a section
index of 0, as the MSVC linker does, instead of asserting.

Author: cjacek

lld/COFF/PDB.cpp

@@ -1135,9 +1135,12 @@ static pdb::BulkPublic createPublic(COFFLinkerContext &ctx, Defined *def) {
   pub.setFlags(flags);
 
   OutputSection *os = ctx.getOutputSection(def->getChunk());
-  assert(os && "all publics should be in final image");
-  pub.Offset = def->getRVA() - os->getRVA();
-  pub.Segment = os->sectionIndex;
+  assert((os || !def->getChunk()->getSize()) &&
+         "all publics should be in final image");
+  if (os) {
+    pub.Offset = def->getRVA() - os->getRVA();
+    pub.Segment = os->sectionIndex;
+  }
   return pub;
 }
 

lld/test/COFF/pdb-empty-sec.s

@@ -0,0 +1,19 @@
+// REQUIRES: x86
+
+// RUN: llvm-mc -filetype=obj -triple=x86_64-windows %s -o %t.obj
+// RUN: lld-link -dll -noentry -debug %t.obj -out:%t.dll
+// RUN: llvm-pdbutil dump -publics %t.pdb | FileCheck %s
+
+// CHECK:       Records
+// CHECK-NEXT:       0 | S_PUB32 [size = 20] `func`
+// CHECK-NEXT:           flags = none, addr = 0001:0000
+// CHECK-NEXT:      20 | S_PUB32 [size = 20] `sym`
+// CHECK-NEXT:           flags = none, addr = 0000:0000
+
+        .globl sym
+        .data
+sym:
+        .text
+        .globl func
+func:
+        ret