[MC] [COFF] Make sure that weak external symbols are undefined symbols

For comdats (e.g. caused by -ffunction-sections), Section is already
set here; make sure it's null, for the weak external symbol to be undefined.

This fixes PR46779.

Differential Revision: https://reviews.llvm.org/D84507

llvm/lib/MC/WinCOFFObjectWriter.cpp

@@ -375,6 +375,7 @@ void WinCOFFObjectWriter::DefineSymbol(const MCSymbol &MCSym,
   COFFSymbol *Local = nullptr;
   if (cast<MCSymbolCOFF>(MCSym).isWeakExternal()) {
     Sym->Data.StorageClass = COFF::IMAGE_SYM_CLASS_WEAK_EXTERNAL;
+    Sym->Section = nullptr;
 
     COFFSymbol *WeakDefault = getLinkedSymbol(MCSym);
     if (!WeakDefault) {

llvm/test/MC/COFF/weak-comdat.s

@@ -0,0 +1,34 @@
+// RUN: llvm-mc -filetype=obj -triple x86_64-pc-win32 %s -o %t.o
+// RUN: llvm-readobj --symbols %t.o | FileCheck %s
+
+// Test that the weak symbol is properly undefined, while originally being
+// the leader symbol for a comdat. (This can easily happen if building with
+// -ffunction-sections).
+
+        .section .text$func,"xr",one_only,func
+        .weak   func
+func:
+        ret
+
+// CHECK:       Symbol {
+// CHECK:         Name: func
+// CHECK-NEXT:    Value: 0
+// CHECK-NEXT:    Section: IMAGE_SYM_UNDEFINED (0)
+// CHECK-NEXT:    BaseType: Null (0x0)
+// CHECK-NEXT:    ComplexType: Null (0x0)
+// CHECK-NEXT:    StorageClass: WeakExternal (0x69)
+// CHECK-NEXT:    AuxSymbolCount: 1
+// CHECK-NEXT:    AuxWeakExternal {
+// CHECK-NEXT:      Linked: .weak.func.default (10)
+// CHECK-NEXT:      Search: Alias (0x3)
+// CHECK-NEXT:    }
+// CHECK-NEXT:  }
+// CHECK-NEXT:  Symbol {
+// CHECK-NEXT:    Name: .weak.func.default
+// CHECK-NEXT:    Value: 0
+// CHECK-NEXT:    Section: .text$func (4)
+// CHECK-NEXT:    BaseType: Null (0x0)
+// CHECK-NEXT:    ComplexType: Null (0x0)
+// CHECK-NEXT:    StorageClass: External (0x2)
+// CHECK-NEXT:    AuxSymbolCount: 0
+// CHECK-NEXT:  }