Address review comments

Author: aschwaighofer

lib/IRGen/AllocStackHoisting.cpp

@@ -332,7 +332,7 @@ class HoistAllocStack {
 };
 } // end anonymous namespace
 
-bool indicatesAvailabilityMacroUse(SILInstruction *I) {
+bool indicatesDynamicAvailabilityCheckUse(SILInstruction *I) {
   auto *Apply = dyn_cast<ApplyInst>(I);
   if (!Apply)
     return false;
@@ -363,7 +363,7 @@ void HoistAllocStack::collectHoistableInstructions() {
         continue;
       }
       // Don't perform alloc_stack hoisting in functions with availability.
-      if (indicatesAvailabilityMacroUse(&Inst)) {
+      if (indicatesDynamicAvailabilityCheckUse(&Inst)) {
         AllocStackToHoist.clear();
         return;
       }

stdlib/public/core/Availability.swift

@@ -25,6 +25,9 @@ public func _stdlib_isOSVersionAtLeast(
   _ patch: Builtin.Word
 ) -> Builtin.Int1 {
 #if os(macOS) || os(iOS) || os(tvOS) || os(watchOS)
+  // The call to _swift_stdlib_operatingSystemVersion is used as an indicator
+  // that this function was called by a compiler optimization pass. If it is
+  // replaced that pass needs to be updated.
   let runningVersion = _swift_stdlib_operatingSystemVersion()
   let queryVersion = _SwiftNSOperatingSystemVersion(
     majorVersion: Int(major),