[APIGen] Handle SPI availability for nested decls (#85048)

- Add SPI availability information to `APIAvailability` from attribute
`@_spi_available`.
- Correctly obtain effective availability for nested declarations
without immediate availability attributes.

Resolves rdar://159702280

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test/APIJSON/struct.swift

@@ -28,7 +28,8 @@ public struct TestStruct {
 // CHECK-NEXT:       "name": "_$s8MyModule10TestStructVACycfC",
 // CHECK-NEXT:       "access": "public",
 // CHECK-NEXT:       "file": "SOURCE_DIR/test/APIJSON/struct.swift",
-// CHECK-NEXT:       "linkage": "exported"
+// CHECK-NEXT:       "linkage": "exported",
+// CHECK-NEXT:       "introduced": "10.13"
 // CHECK-NEXT:     },
 // CHECK-NEXT:     {
 // CHECK-NEXT:       "name": "_$s8MyModule10TestStructVMa",