[LLD] [COFF] Always include the size of the string table size field

[mstorsjo]

Summary

Even if we don't actually write any string table contents, the 4 byte size for the string table will always be written. Make sure we accommodate for this in the file size. Since this size is aligned up, this would seldom be an issue in practice.

[ruiu]

LGTM, but please add a testcase before committing.

[mstorsjo]

Here's a testcase for this issue. In practice it's clearly pathological, but still - with this testcase, LLD would earlier output a broken file, as detected by both llvm-readobj and dumpbin.exe. Will commit in a while since it's already approved.

COFF/Writer.cpp

void Writer::createSymbolAndStringTable() {
   OutputSection *LastSection = OutputSections.back();
   // We position the symbol table to be adjacent to the end of the last section.
   uint64_t FileOff = LastSection->getFileOff() +
                      alignTo(LastSection->getRawSize(), SectorSize);
   if (!OutputSymtab.empty()) {
     PointerToSymbolTable = FileOff;
     FileOff += OutputSymtab.size() * sizeof(coff_symbol16);
   }
-  if (!Strtab.empty())
-    FileOff += Strtab.size() + 4;
   FileSize = alignTo(FileOff, SectorSize);
 }
 
 // Visits all sections to assign incremental, non-overlapping RVAs and
 // file offsets.
 void Writer::assignAddresses() {
   SizeOfHeaders = DOSStubSize + sizeof(PEMagic) + sizeof(coff_file_header) +
                   sizeof(data_directory) * NumberfOfDataDirectory +