SIL: Remove linkage parameter from SILModule::createWitnessTableDeclaration()

All callers were doing the same thing here, so move it inside the
function. Also, change getRootNormalConformance(), which is deprecated,
to getRootConformance().

include/swift/SIL/SILModule.h

@@ -572,8 +572,7 @@ public:
 
   // Given a protocol conformance, attempt to create a witness table declaration
   // for it.
-  SILWitnessTable *
-  createWitnessTableDeclaration(ProtocolConformance *C, SILLinkage linkage);
+  SILWitnessTable *createWitnessTableDeclaration(const ProtocolConformance *C);
 
   // Given a protocol, attempt to create a default witness table declaration
   // for it.

lib/SIL/Linker.cpp

@@ -217,9 +217,7 @@ void SILLinkerVisitor::visitProtocolConformance(
   // If we don't find any witness table for the conformance, bail and return
   // false.
   if (!WT) {
-    Mod.createWitnessTableDeclaration(
-        C, getLinkageForProtocolConformance(
-               C->getRootNormalConformance(), NotForDefinition));
+    Mod.createWitnessTableDeclaration(C);
 
     // Adding the declaration may allow us to now deserialize the body.
     // Force the body if we must deserialize this witness table.

lib/SIL/SILInstructions.cpp

@@ -317,9 +317,7 @@ static void declareWitnessTable(SILModule &Mod,
   if (conformanceRef.isAbstract()) return;
   auto C = conformanceRef.getConcrete();
   if (!Mod.lookUpWitnessTable(C, false))
-    Mod.createWitnessTableDeclaration(C,
-        getLinkageForProtocolConformance(C->getRootNormalConformance(),
-                                         NotForDefinition));
+    Mod.createWitnessTableDeclaration(C);
 }
 
 AllocExistentialBoxInst *AllocExistentialBoxInst::create(

lib/SIL/SILModule.cpp

@@ -144,8 +144,7 @@ void SILModule::deallocateInst(SILInstruction *I) {
 }
 
 SILWitnessTable *
-SILModule::createWitnessTableDeclaration(ProtocolConformance *C,
-                                         SILLinkage linkage) {
+SILModule::createWitnessTableDeclaration(const ProtocolConformance *C) {
   // If we are passed in a null conformance (a valid value), just return nullptr
   // since we cannot map a witness table to it.
   if (!C)
@@ -153,7 +152,9 @@ SILModule::createWitnessTableDeclaration(ProtocolConformance *C,
 
   // Extract the root conformance.
   auto rootC = C->getRootConformance();
-  return SILWitnessTable::create(*this, linkage, rootC);
+  auto linkage = getLinkageForProtocolConformance(rootC, NotForDefinition);
+  return SILWitnessTable::create(*this, linkage,
+                                 const_cast<RootProtocolConformance *>(rootC));
 }
 
 SILWitnessTable *