[rbi] Remove support for sending result for impl-function-type in the old mangler.

docs/ABI/Mangling.rst

@@ -873,6 +873,9 @@ mangled in to disambiguate.
   #if SWIFT_RUNTIME_VERSION >= 5.5
     SENDABLE ::= 'h'                         // @Sendable
     ASYNC ::= 'H'                            // @async
+  #endif
+
+  #if SWIFT_RUNTIME_VERSION >= 6.0
     SENDING-RESULT ::= 'T'                   // sending result
   #endif
 

lib/Demangling/OldDemangler.cpp

@@ -2252,9 +2252,6 @@ private:
     if (Mangled.nextIf('H'))
       addImplFunctionAttribute(type, "@async");
 
-    if (Mangled.nextIf('T'))
-      addImplFunctionAttribute(type, "sending-result");
-
     // Enter a new generic context if this type is generic.
     // FIXME: replace with std::optional, when we have it.
     bool isPseudogeneric = false;

lib/Demangling/Remangler.cpp

@@ -2122,7 +2122,6 @@ ManglingError Remangler::mangleImplFunctionType(Node *node, unsigned depth) {
         char FuncAttr = llvm::StringSwitch<char>(Child->getText())
                             .Case("@Sendable", 'h')
                             .Case("@async", 'H')
-                            .Case("sending-result", 'T')
                             .Default(0);
         if (!FuncAttr) {
           return MANGLING_ERROR(ManglingError::InvalidImplFunctionAttribute,