[BOLT] Check MCContext errors

[Amir]

Summary

Abort on emission errors to prevent a malformed binary being written.
Example:

<unknown>:0: error: Undefined temporary symbol .Ltmp26310
<unknown>:0: error: Undefined temporary symbol .Ltmp26311
<unknown>:0: error: Undefined temporary symbol .Ltmp26312
<unknown>:0: error: Undefined temporary symbol .Ltmp26313
<unknown>:0: error: Undefined temporary symbol .Ltmp26314
<unknown>:0: error: Undefined temporary symbol .Ltmp26315
BOLT-ERROR: Emission failed.

bolt/lib/Rewrite/RewriteInstance.cpp

if (opts::UseOldText || opts::StrictMode) {
       // Make .eh_frame relocatable.
       relocateEHFrameSection();
     }
   }
 
   emitBinaryContext(*Streamer, *BC, getOrgSecPrefix());
 
   Streamer->Finish();
+  if (Streamer->getContext().hadError()) {
+    errs() << "BOLT-ERROR: Emission failed.\n";
+    exit(1);
+  }
 
   //////////////////////////////////////////////////////////////////////////////
   // Assign addresses to new sections.
   //////////////////////////////////////////////////////////////////////////////
 
   // Get output object as ObjectFile.
   std::unique_ptr<MemoryBuffer> ObjectMemBuffer =
       MemoryBuffer::getMemBuffer(BOS->str(), "in-memory object file", false);
void RewriteInstance::emitAndLink() {
   // Assign addresses to all sections. If key corresponds to the object
   // created by ourselves, call our regular mapping function. If we are
   // loading additional objects as part of runtime libraries for
   // instrumentation, treat them as extra sections.
   mapFileSections(*RTDyld);
 
   RTDyld->finalizeWithMemoryManagerLocking();
   if (RTDyld->hasError()) {
-    outs() << "BOLT-ERROR: RTDyld failed: " << RTDyld->getErrorString() << "\n";
+    errs() << "BOLT-ERROR: RTDyld failed: " << RTDyld->getErrorString() << "\n";
     exit(1);
   }
 
   // Update output addresses based on the new section map and
   // layout. Only do this for the object created by ourselves.
   updateOutputValues(FinalLayout);
 
   if (opts::UpdateDebugSections)