[AsmPrinter] Use the McASMInfo to determine if we need descriptors.

[sfertile]

Summary

In https://reviews.llvm.org/rG8b737688c21a9755cae14cb9343930e0882164ab I switched the condition gating the creation of the descriptor symbol from
checking the MCAsmInfo if we need to support descriptors, to if the OS was AIX. Technically the 2 should be interchangeable: if we are
targeting AIX then we need to emit XCOFF object files, and the MCAsmInfo must return true for needing function descriptors.

This doesn't account for lit test with runsteps that only set the arch. Eg: test/CodeGen/XCore/section-name.ll
which when run natively on AIX we end up with a target xcore-ibm-aix and needFunctionDescriptors is false.

This patch reverts to using the MCAsmInfo and adds an assert that the target OS must be AIX since that is the only target using the descriptor hook.

[daltenty]

LGTM

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

MCSymbol *AsmPrinter::getCurExceptionSym() {
   return CurExceptionSym;
 }
 
 void AsmPrinter::SetupMachineFunction(MachineFunction &MF) {
   this->MF = &MF;
   const Function &F = MF.getFunction();
 
   // Get the function symbol.
-  if (TM.getTargetTriple().isOSAIX()) {
+  if (!MAI->needsFunctionDescriptors()) {
+    CurrentFnSym = getSymbol(&MF.getFunction());
+  } else {
+    assert(TM.getTargetTriple().isOSAIX() &&
+           "Only AIX uses the function descriptor hooks.");
     // AIX is unique here in that the name of the symbol emitted for the
     // function body does not have the same name as the source function's
     // C-linkage name.
-    assert(MAI->needsFunctionDescriptors() && "AIX ABI is descriptor based.");
     assert(CurrentFnDescSym && "The function descriptor symbol needs to be"
-                               " initalized first.");
+                               " initalized first.");
 
     // Get the function entry point symbol.
     CurrentFnSym =
         OutContext.getOrCreateSymbol("." + CurrentFnDescSym->getName());
 
     // Set the containing csect.
     MCSectionXCOFF *FnEntryPointSec =
         cast<MCSectionXCOFF>(getObjFileLowering().SectionForGlobal(&F, TM));
     cast<MCSymbolXCOFF>(CurrentFnSym)->setContainingCsect(FnEntryPointSec);
-  } else {
-    CurrentFnSym = getSymbol(&MF.getFunction());
   }
 
   CurrentFnSymForSize = CurrentFnSym;
   CurrentFnBegin = nullptr;
   CurExceptionSym = nullptr;
   bool NeedsLocalForSize = MAI->needsLocalForSize();
   if (F.hasFnAttribute("patchable-function-entry") ||
       needFuncLabelsForEHOrDebugInfo(MF, MMI) || NeedsLocalForSize ||