sprinkle some llvm_unreachable for MSVC (NFC)

MSVC does not realize that the switch is exhaustive and requires that
the path is explicitly marked as unreachable.  This silences the C4715
warning ("not all control paths return a value").

Author: compnerd

include/swift/SIL/ApplySite.h

@@ -519,6 +519,7 @@ class FullApplySite : public ApplySite {
     case FullApplySiteKind::BeginApplyInst:
       return cast<BeginApplyInst>(getInstruction())->getInoutArguments();
     }
+    llvm_unreachable("invalid apply kind");
   }
 
   /// Returns true if \p op is the callee operand of this apply site

include/swift/SIL/SILDifferentiabilityWitness.h

@@ -113,6 +113,7 @@ class SILDifferentiabilityWitness
     case AutoDiffDerivativeFunctionKind::VJP:
       return VJP;
     }
+    llvm_unreachable("invalid derivative type");
   }
   void setJVP(SILFunction *jvp) { JVP = jvp; }
   void setVJP(SILFunction *vjp) { VJP = vjp; }

include/swift/SIL/SILInstruction.h

@@ -8107,6 +8107,7 @@ class DifferentiableFunctionInst final
     case AutoDiffDerivativeFunctionKind::VJP:
       return getVJPFunction();
     }
+    llvm_unreachable("invalid derivative kind");
   }
 };
 

include/swift/SILOptimizer/Utils/Differentiation/ADContext.h

@@ -374,6 +374,7 @@ ADContext::emitNondifferentiabilityError(SourceLoc loc,
     return diagnose(loc, diag::autodiff_when_differentiating_function_call);
   }
   }
+  llvm_unreachable("invalid invoker");
 }
 
 } // end namespace autodiff

lib/FrontendTool/FrontendTool.cpp

@@ -1462,6 +1462,7 @@ static bool validateTBDIfNeeded(const CompilerInvocation &Invocation,
     case FrontendOptions::TBDValidationMode::All:
       return true;
     }
+    llvm_unreachable("invalid mode");
   }();
 
   TBDGenOptions Opts = Invocation.getTBDGenOptions();

lib/IRGen/GenDiffFunc.cpp

@@ -65,6 +65,7 @@ class DifferentiableFuncFieldInfo final
     case NormalDifferentiableFunctionTypeComponent::VJP:
       return "vjp";
     }
+    llvm_unreachable("invalid component type");
   }
 
   SILType getType(IRGenModule &IGM, SILType t) const {
@@ -209,6 +210,7 @@ class LinearFuncFieldInfo final : public RecordField<LinearFuncFieldInfo> {
     case LinearDifferentiableFunctionTypeComponent::Transpose:
       return "transpose";
     }
+    llvm_unreachable("invalid component type");
   }
 
   SILType getType(IRGenModule &IGM, SILType t) const {
@@ -223,6 +225,7 @@ class LinearFuncFieldInfo final : public RecordField<LinearFuncFieldInfo> {
           LookUpConformanceInModule(IGM.getSwiftModule()));
       return SILType::getPrimitiveObjectType(transposeTy);
     }
+    llvm_unreachable("invalid component type");
   }
 };
 
@@ -321,6 +324,7 @@ class LinearFuncTypeBuilder
           LookUpConformanceInModule(IGM.getSwiftModule()));
       return SILType::getPrimitiveObjectType(transposeTy);
     }
+    llvm_unreachable("invalid component type");
   }
 
   StructLayout performLayout(ArrayRef<const TypeInfo *> fieldTypes) {

lib/IRGen/GenPointerAuth.cpp

@@ -584,6 +584,7 @@ PointerAuthEntity::getTypeDiscriminator(IRGenModule &IGM) const {
       llvm_unreachable("not type discriminated");
     }
     }
+    llvm_unreachable("invalid representation");
   };
 
   auto getCoroutineYieldTypesDiscriminator = [&](CanSILFunctionType fnType) {

lib/IRGen/Linking.cpp

@@ -738,6 +738,7 @@ bool LinkEntity::isContextDescriptor() const {
   case Kind::DifferentiabilityWitness:
     return false;
   }
+  llvm_unreachable("invalid descriptor");
 }
 
 llvm::Type *LinkEntity::getDefaultDeclarationType(IRGenModule &IGM) const {
@@ -1088,6 +1089,7 @@ DeclContext *LinkEntity::getDeclContextForEmission() const {
   case Kind::DifferentiabilityWitness:
     return nullptr;
   }
+  llvm_unreachable("invalid decl kind");
 }
 
 bool LinkEntity::isAlwaysSharedLinkage() const {

lib/SIL/IR/AbstractionPattern.cpp

@@ -927,6 +927,7 @@ const {
 
     return AbstractionPattern(getGenericSignature(), memberTy);
   }
+  llvm_unreachable("invalid abstraction pattern kind");
 }
 
 AbstractionPattern AbstractionPattern::getAutoDiffDerivativeFunctionType(

lib/SIL/IR/SILInstructions.cpp

@@ -751,6 +751,7 @@ getExtracteeType(
         LookUpConformanceInModule(module.getSwiftModule()));
     return SILType::getPrimitiveObjectType(transposeFnTy);
   }
+  llvm_unreachable("invalid extractee");
 }
 
 LinearFunctionExtractInst::LinearFunctionExtractInst(