Add opt remarks to Generic Specializer pass

Adds a combined API to output both debug message and optimization remarks.

The previously added test partial_specialization_debug.sil ensures that it's an
NFC for debug output.

lib/SILOptimizer/Transforms/GenericSpecializer.cpp

@@ -17,6 +17,7 @@
 
 #define DEBUG_TYPE "sil-generic-specializer"
 
+#include "swift/SIL/OptimizationRemark.h"
 #include "swift/SIL/SILFunction.h"
 #include "swift/SIL/SILInstruction.h"
 #include "swift/SILOptimizer/Utils/Generics.h"
@@ -49,6 +50,7 @@ class GenericSpecializer : public SILFunctionTransform {
 bool GenericSpecializer::specializeAppliesInFunction(SILFunction &F) {
   DeadInstructionSet DeadApplies;
   llvm::SmallSetVector<SILInstruction *, 8> Applies;
+  OptRemark::Emitter ORE(DEBUG_TYPE, F.getModule());
 
   bool Changed = false;
   for (auto &BB : F) {
@@ -67,8 +69,17 @@ bool GenericSpecializer::specializeAppliesInFunction(SILFunction &F) {
         continue;
 
       auto *Callee = Apply.getReferencedFunction();
-      if (!Callee || !Callee->isDefinition())
+      if (!Callee)
         continue;
+      if (!Callee->isDefinition()) {
+        ORE.emit([&]() {
+          using namespace OptRemark;
+          return RemarkMissed("NoDef", *I)
+                 << "Unable to specialize generic function "
+                 << NV("Callee", Callee) << " since definition is not visible";
+        });
+        continue;
+      }
 
       Applies.insert(Apply.getInstruction());
     }
@@ -87,7 +98,7 @@ bool GenericSpecializer::specializeAppliesInFunction(SILFunction &F) {
       // We have a call that can potentially be specialized, so
       // attempt to do so.
       llvm::SmallVector<SILFunction *, 2> NewFunctions;
-      trySpecializeApplyOfGeneric(Apply, DeadApplies, NewFunctions);
+      trySpecializeApplyOfGeneric(Apply, DeadApplies, NewFunctions, ORE);
 
       // Remove all the now-dead applies. We must do this immediately
       // rather than defer it in order to avoid problems with cloning