Add test case for #56636 / rdar://problem/74794074

Author: slavapestov

test/ClangImporter/Inputs/issue-56636.h

@@ -0,0 +1,6 @@
+@import Foundation;
+
+typedef NS_ENUM( uint32_t, Letter ) {
+  a,
+  b,
+};

test/ClangImporter/issue-56636.swift

@@ -0,0 +1,11 @@
+// RUN: %target-typecheck-verify-swift -import-objc-header %S/Inputs/issue-56636.h
+
+extension Letter: Strideable {
+  public func distance(to other: Self) -> RawValue.Stride {
+    self.rawValue.distance( to: other.rawValue )
+  }
+
+  public func advanced(by n: RawValue.Stride) -> Self {
+    Self( rawValue: self.rawValue.advanced( by: n ) )!
+  }
+}