Enable EliminateAvailableExternally pass in the LTO pipeline.

[tejohnson]

Summary

For LTO we need to enable this pass in the LTO pipeline,
as it is skipped during the "-flto -c" compile step (when PrepareForLTO is
set).

[rnk]

lgtm

lib/Transforms/IPO/PassManagerBuilder.cpp

559–564

I think you can shorten this to something like:

// Drop bodies of available externally objects to improve GlobalDCE.

[tejohnson]

• Update comments per review.

lib/Transforms/IPO/PassManagerBuilder.cpp

void PassManagerBuilder::addLTOOptimizationPasses(legacy::PassManagerBase &PM) {
   PM.add(createJumpThreadingPass());
 }
 
 void PassManagerBuilder::addLateLTOOptimizationPasses(
     legacy::PassManagerBase &PM) {
   // Delete basic blocks, which optimization passes may have killed.
   PM.add(createCFGSimplificationPass());
 
+  // Drop bodies of available externally objects to improve GlobalDCE.
+  PM.add(createEliminateAvailableExternallyPass());
+
   // Now that we have optimized the program, discard unreachable functions.
   PM.add(createGlobalDCEPass());
 
   // FIXME: this is profitable (for compiler time) to do at -O0 too, but
   // currently it damages debug info.
   if (MergeFunctions)
     PM.add(createMergeFunctionsPass());
 }
 
 void PassManagerBuilder::populateLTOPassManager(legacy::PassManagerBase &PM) {
   if (LibraryInfo)