[AST] Enforce non-null type for value GenericTypeParamType

Make sure we have a value type for a value generic parameter.

Author: hamishknight

lib/AST/Type.cpp

@@ -2153,6 +2153,8 @@ GenericTypeParamType::GenericTypeParamType(GenericTypeParamKind paramKind,
                                            const ASTContext &ctx)
     : SubstitutableType(TypeKind::GenericTypeParam, &ctx, props),
       Decl(nullptr) {
+  ASSERT(!(paramKind == GenericTypeParamKind::Value && !valueType) &&
+         "Value generic parameter must have type");
   IsDecl = false;
   Depth = depth;
   Weight = weight;

lib/Sema/CSGen.cpp

@@ -1746,7 +1746,7 @@ namespace {
 
     Type visitTypeValueExpr(TypeValueExpr *E) {
       auto ty = E->getParamDecl()->getValueType();
-      if (!ty || ty->hasError())
+      if (ty->hasError())
         return recordInvalidNode(E);
       return ty;
     }

lib/Sema/TypeCheckType.cpp

@@ -811,10 +811,9 @@ namespace {
       // If either value type has an error, we've already diagnosed the issue.
       auto paramValueTy = paramType->getValueType();
       auto secondValueTy = secondType->getValueType();
-      if (!paramValueTy || paramValueTy->hasError() ||
-          !secondValueTy || secondValueTy->hasError()) {
+      if (paramValueTy->hasError() || secondValueTy->hasError())
         return true;
-      }
+
       // Otherwise, these are both value parameters and check that both their
       // value types are the same.
       return paramValueTy->isEqual(secondValueTy);