[sil-optimizer] Add LLVM_ATTRIBUTE_UNUSED to InstModCallback::on* methods.

InstModCallback is a value type and as such the original callback struct is not
being modified. Instead, a new InstModCallback struct is returned that is the
old callback + assignment of the passed in callback to the appropriate
field. Thus it makes sense to put this attribute on these methods so that we get
a warning if one does not use the new returned callback (otherwise, why would
one call this method?!). More likely a bug.

Author: gottesmm

include/swift/SILOptimizer/Utils/InstOptUtils.h

@@ -104,23 +104,24 @@ class InstModCallbacks {
   InstModCallbacks(const InstModCallbacks &) = default;
 
   /// Return a copy of self with deleteInstFunc set to \p newDeleteInstFunc.
-  InstModCallbacks onDelete(decltype(deleteInstFunc) newDeleteInstFunc) const {
+  LLVM_ATTRIBUTE_UNUSED InstModCallbacks
+  onDelete(decltype(deleteInstFunc) newDeleteInstFunc) const {
     InstModCallbacks result = *this;
     result.deleteInstFunc = newDeleteInstFunc;
     return result;
   }
 
   /// Return a copy of self with createdNewInstFunc set to \p
   /// newCreatedNewInstFunc.
-  InstModCallbacks
+  LLVM_ATTRIBUTE_UNUSED InstModCallbacks
   onCreateNewInst(decltype(createdNewInstFunc) newCreatedNewInstFunc) const {
     InstModCallbacks result = *this;
     result.createdNewInstFunc = newCreatedNewInstFunc;
     return result;
   }
 
   /// Return a copy of self with setUseValueFunc set to \p newSetUseValueFunc.
-  InstModCallbacks
+  LLVM_ATTRIBUTE_UNUSED InstModCallbacks
   onSetUseValue(decltype(setUseValueFunc) newSetUseValueFunc) const {
     InstModCallbacks result = *this;
     result.setUseValueFunc = newSetUseValueFunc;
@@ -129,6 +130,7 @@ class InstModCallbacks {
 
   /// Return a copy of self with notifyWillBeDeletedFunc set to \p
   /// newNotifyWillBeDeletedFunc.
+  LLVM_ATTRIBUTE_UNUSED
   InstModCallbacks onNotifyWillBeDeleted(
       decltype(notifyWillBeDeletedFunc) newNotifyWillBeDeletedFunc) const {
     InstModCallbacks result = *this;