ASTContext: Tidy up the interface of getOpenedArchetypeSignature

Drop a dead parameter, encapsulate input type canonicalization, and slightly improve the doc comment.

Author: AnthonyLatsis

Diff for: include/swift/AST/ASTContext.h

@@ -1065,9 +1065,8 @@ class ASTContext final {
   CanGenericSignature getSingleGenericParameterSignature() const;
 
   /// Retrieve a generic signature with a single type parameter conforming
-  /// to the given opened archetype.
-  CanGenericSignature getOpenedArchetypeSignature(CanType existential,
-                                                  ModuleDecl *mod);
+  /// to the given protocol or composition type, like <T: type>.
+  CanGenericSignature getOpenedArchetypeSignature(Type type);
 
   GenericSignature getOverrideGenericSignature(const ValueDecl *base,
                                                const ValueDecl *derived);

Diff for: lib/AST/ASTContext.cpp

@@ -3937,8 +3937,7 @@ GenericEnvironment *OpenedArchetypeType::getGenericEnvironment() const {
   auto thisType = Type(const_cast<OpenedArchetypeType*>(this));
   auto &ctx = thisType->getASTContext();
   // Create a generic environment to represent the opened type.
-  auto signature =
-      ctx.getOpenedArchetypeSignature(Opened->getCanonicalType(), nullptr);
+  auto signature = ctx.getOpenedArchetypeSignature(Opened);
   auto *builder = signature->getGenericSignatureBuilder();
   auto *env = GenericEnvironment::getIncomplete(signature, builder);
   env->addMapping(signature->getGenericParams()[0], thisType);
@@ -4567,9 +4566,10 @@ CanGenericSignature ASTContext::getSingleGenericParameterSignature() const {
 // Type::getExistentialLayout()). In particular, the opened archetype signature
 // does not have requirements for conformances inherited from superclass
 // constraints while existential values do.
-CanGenericSignature ASTContext::getOpenedArchetypeSignature(CanType existential,
-                                                            ModuleDecl *mod) {
-  assert(existential.isExistentialType());
+CanGenericSignature ASTContext::getOpenedArchetypeSignature(Type type) {
+  assert(type->isExistentialType());
+
+  const CanType existential = type->getCanonicalType();
 
   // The opened archetype signature for a protocol type is identical
   // to the protocol's own canonical generic signature.

Diff for: lib/SILOptimizer/Utils/Existential.cpp

@@ -255,8 +255,7 @@ void ConcreteExistentialInfo::initializeSubstitutionMap(
   // than their corresponding existential, ExistentialConformances needs to be
   // filtered when using it with this (phony) generic signature.
   CanGenericSignature ExistentialSig =
-      M->getASTContext().getOpenedArchetypeSignature(ExistentialType,
-                                                     M->getSwiftModule());
+      M->getASTContext().getOpenedArchetypeSignature(ExistentialType);
   ExistentialSubs = SubstitutionMap::get(
       ExistentialSig, [&](SubstitutableType *type) { return ConcreteType; },
       [&](CanType /*depType*/, Type /*replaceType*/,