Merge pull request swiftlang#16854 from dcci/followups

Author: swift-ci

include/swift/SIL/SILLocation.h

@@ -428,7 +428,7 @@ class SILLocation {
   static DebugLoc decode(SourceLoc Loc, const SourceManager &SM);
 
   /// Return the decoded debug location.
-  DebugLoc decodeDebugLoc(const SourceManager &SM) const {
+  LLVM_NODISCARD DebugLoc decodeDebugLoc(const SourceManager &SM) const {
     return isDebugInfoLoc() ? Loc.DebugInfoLoc
                             : decode(getDebugSourceLoc(), SM);
   }

lib/IRGen/IRGenSIL.cpp

@@ -737,6 +737,8 @@ class IRGenSILFunction :
       return;
 
     llvm::IRBuilder<> ZeroInitBuilder(AI->getNextNode());
+
+    // No debug location is how LLVM marks prologue instructions.
     ZeroInitBuilder.SetCurrentDebugLocation(nullptr);
     auto *BC =
         ZeroInitBuilder.CreateBitCast(AI, IGM.OpaquePtrTy->getPointerTo());