KeyPaths: Support instantiating generic property descriptors from concrete contexts.

This is a bit easier than the fully general case where both the external descriptor and local pattern have captured arguments (because of generics or subscript indices) since we don't have to combine the two argument files in one component.

Author: jckarter

stdlib/public/SwiftShims/KeyPath.h

@@ -98,6 +98,11 @@ static const __swift_uint32_t _SwiftKeyPathComponentHeader_ComputedIDResolved
 static const __swift_uint32_t _SwiftKeyPathComponentHeader_ComputedIDUnresolvedIndirectPointer
   = 0x00000002U;
 
+extern void *(swift_keyPathGenericWitnessTable[]);
+
+static inline void *__swift_keyPathGenericWitnessTable_addr(void) {
+  return swift_keyPathGenericWitnessTable;
+}
 
 #ifdef __cplusplus
 } // extern "C"