[LLD] [COFF] [RFC] Don't exit the process while writing section data in parallel

[mstorsjo]

Summary

This is an alternative solution to PR38647.

[mstorsjo]

We went with D51095 instead.

COFF/Writer.cpp

 }
 
 // Write section contents to a mmap'ed file.
 void Writer::writeSections() {
   // Record the number of sections to apply section index relocations
   // against absolute symbols. See applySecIdx in Chunks.cpp..
   DefinedAbsolute::NumOutputSections = OutputSections.size();
 
+  bool OrigExitEarly = errorHandler().ExitEarly;
+  // Don't exit when reaching the error count limit while running multiple
+  // parallel threads; the thread that exits will try to close the output buffer
+  // that other threads still may be touching.
+  errorHandler().ExitEarly = false;
+
   uint8_t *Buf = Buffer->getBufferStart();
   for (OutputSection *Sec : OutputSections) {
     uint8_t *SecBuf = Buf + Sec->getFileOff();
     // Fill gaps between functions in .text with INT3 instructions
     // instead of leaving as NUL bytes (which can be interpreted as
     // ADD instructions).
     if (Sec->Header.Characteristics & IMAGE_SCN_CNT_CODE)
       memset(SecBuf, 0xCC, Sec->getRawSize());
     for_each(parallel::par, Sec->getChunks().begin(), Sec->getChunks().end(),
              [&](Chunk *C) { C->writeTo(SecBuf); });
   }
+
+  errorHandler().ExitEarly = OrigExitEarly;
+  if (errorHandler().ExitEarly &&
+      errorHandler().ErrorCount >= errorHandler().ErrorLimit)
+    exitLld(1);
 }
 
 void Writer::writeBuildId() {
   // There are two important parts to the build ID.
   // 1) If building with debug info, the COFF debug directory contains a
   //    timestamp as well as a Guid and Age of the PDB.
   // 2) In all cases, the PE COFF file header also contains a timestamp.
   // For reproducibility, instead of a timestamp we want to use a hash of the