[AST] Remove asserts from getBuiltinInitDecl

Allow querying for types that may not conform. Failures here should
be asserted downstream (e.g in `setBuiltinInitializer`).

Author: hamishknight

lib/AST/ASTContext.cpp

@@ -1698,15 +1698,13 @@ ASTContext::getBuiltinInitDecl(NominalTypeDecl *decl,
   auto builtinProtocol = getProtocol(builtinProtocolKind);
   auto builtinConformance = lookupConformance(type, builtinProtocol);
   if (builtinConformance.isInvalid()) {
-    assert(false && "Missing required conformance");
     witness = ConcreteDeclRef();
     return witness;
   }
 
   auto *ctx = const_cast<ASTContext *>(this);
   witness = builtinConformance.getWitnessByName(initName(*ctx));
   if (!witness) {
-    assert(false && "Missing required witness");
     witness = ConcreteDeclRef();
     return witness;
   }