[lld] COFF: Fix bug involving archives defining a symbol multiple times.

[pcc]

Summary

Previously we were unnecessarily loading lazy symbols if they appeared in an
archive multiple times, as can happen with comdat symbols. This change fixes
the bug by only loading symbols from archives at load time if the original
symbol was undefined.

[ruiu]

LGTM

COFF/SymbolTable.cpp

for (;;) {
     if (isa<Defined>(Existing))
       return;
     if (Lazy *L = dyn_cast<Lazy>(Existing))
       if (L->getFileIndex() < New->getFileIndex())
         return;
     if (!Sym->Body.compare_exchange_strong(Existing, New))
       continue;
     New->setBackref(Sym);
+    if (isa<Undefined>(Existing))
       Accum->push_back(Sym);
     return;
   }
 }
 
 std::error_code SymbolTable::addSymbol(SymbolBody *New) {
   // Find an existing symbol or create and insert a new one.
   assert(isa<Defined>(New) || isa<Undefined>(New));
   Symbol *Sym = insert(New);

test/COFF/ar-comdat.test

+# RUN: yaml2obj %s > %t1.obj
+# RUN: yaml2obj %s > %t2.obj
+# RUN: llvm-lib /out:%t.lib %t1.obj %t2.obj
+# RUN: lld -flavor link2 /out:%t.exe /lldmap:%t.map /entry:main /subsystem:console %p/Inputs/ret42.obj %t.lib
+# RUN: FileCheck %s < %t.map
+
+# CHECK-NOT: .lib
+
+---
+header:          
+  Machine:         IMAGE_FILE_MACHINE_AMD64
+  Characteristics: [  ]
+sections:        
+  - Name:            .bss
+    Characteristics: [ IMAGE_SCN_CNT_UNINITIALIZED_DATA, IMAGE_SCN_LNK_COMDAT, IMAGE_SCN_MEM_READ, IMAGE_SCN_MEM_WRITE ]
+    Alignment:       4
+    SectionData:     ''
+symbols:         
+  - Name:            .bss
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition: 
+      Length:          4
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        0
+      Number:          1
+      Selection:       IMAGE_COMDAT_SELECT_ANY
+  - Name:            x
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+...