[COFF] Unify output section code. NFC

[smeenai]

Summary

Peter Collingbourne suggested moving the switch to the top of the
function, so that all the code that cares about the output section for a
symbol is in the same place.

[pcc]

COFF/Writer.cpp

618

You can move the if on line 606 into the switch as well.

627

I'd drop the assert here and combine line 619 and 621 into one.

[smeenai]

@pcc's comments

[pcc]

COFF/Writer.cpp

613

I meant that you can write:

case Symbol::DefinedSyntheticKind:
  return None;

[smeenai]

@pcc comment

[pcc]

LGTM

COFF/Writer.cpp

size_t Writer::addEntryToStringTable(StringRef Str) {
   assert(Str.size() > COFF::NameSize);
   size_t OffsetOfEntry = Strtab.size() + 4; // +4 for the size field
   Strtab.insert(Strtab.end(), Str.begin(), Str.end());
   Strtab.push_back('\0');
   return OffsetOfEntry;
 }
 
 Optional<coff_symbol16> Writer::createSymbol(Defined *Def) {
+  coff_symbol16 Sym;
+  switch (Def->kind()) {
+  case Symbol::DefinedAbsoluteKind:
+    Sym.Value = Def->getRVA();
+    Sym.SectionNumber = IMAGE_SYM_ABSOLUTE;
+    break;
+  default: {
     // Relative symbols are unrepresentable in a COFF symbol table.
     if (isa<DefinedSynthetic>(Def))
       return None;
 
     // Don't write symbols that won't be written to the output to the symbol
     // table.
-  OutputSection *OS = nullptr;
-  if (Chunk *C = Def->getChunk()) {
-    OS = C->getOutputSection();
+    OutputSection *OS = Def->getChunk()->getOutputSection();
     if (!OS)
       return None;
+
+    Sym.Value = Def->getRVA() - OS->getRVA();
+    Sym.SectionNumber = OS->SectionIndex;
+    break;
+  }
   }
 
-  coff_symbol16 Sym;
   StringRef Name = Def->getName();
   if (Name.size() > COFF::NameSize) {
     Sym.Name.Offset.Zeroes = 0;
     Sym.Name.Offset.Offset = addEntryToStringTable(Name);
   } else {
     memset(Sym.Name.ShortName, 0, COFF::NameSize);
     memcpy(Sym.Name.ShortName, Name.data(), Name.size());
   }
 
   if (auto *D = dyn_cast<DefinedCOFF>(Def)) {
     COFFSymbolRef Ref = D->getCOFFSymbol();
     Sym.Type = Ref.getType();
     Sym.StorageClass = Ref.getStorageClass();
   } else {
     Sym.Type = IMAGE_SYM_TYPE_NULL;
     Sym.StorageClass = IMAGE_SYM_CLASS_EXTERNAL;
   }
   Sym.NumberOfAuxSymbols = 0;
-
-  switch (Def->kind()) {
-  case Symbol::DefinedAbsoluteKind:
-    Sym.Value = Def->getRVA();
-    Sym.SectionNumber = IMAGE_SYM_ABSOLUTE;
-    break;
-  default: {
-    assert(OS && "Writing dead symbol to symbol table");
-    Sym.Value = Def->getRVA() - OS->getRVA();
-    Sym.SectionNumber = OS->SectionIndex;
-    break;
-  }
-  }
   return Sym;
 }
 
 void Writer::createSymbolAndStringTable() {
   // PE/COFF images are limited to 8 byte section names. Longer names can be
   // supported by writing a non-standard string table, but this string table is
   // not mapped at runtime and the long names will therefore be inaccessible.
   // link.exe always truncates section names to 8 bytes, whereas binutils always