SR-5829: Improve code conditions and make tests more explicit

(cherry picked from commit 908ffcb)

Author: spevans

Foundation/NSString.swift

@@ -449,12 +449,10 @@ extension NSString {
             guard let otherCh = otherIterator.next() else { break }
 
             if ch != otherCh {
-                if !caseInsensitive || (String(otherCh).lowercased() != String(ch).lowercased()) {
-                    break
-                }
+                guard caseInsensitive && String(ch).lowercased() == String(otherCh).lowercased() else { break }
             }
 
-            if literal, otherCh.unicodeScalars.count != ch.unicodeScalars.count { break }
+            if literal && otherCh.unicodeScalars.count != ch.unicodeScalars.count { break }
             result.append(ch)
         }
 

TestFoundation/TestNSString.swift

@@ -1217,10 +1217,13 @@ class TestNSString: LoopbackServerTest {
         XCTAssertEqual("abcba".commonPrefix(with: "abcde"), "abc")
         XCTAssertEqual("/path/to/file1".commonPrefix(with: "/path/to/file2"), "/path/to/file")
         XCTAssertEqual("/a_really_long_path/to/a/file".commonPrefix(with: "/a_really_long_path/to/the/file"), "/a_really_long_path/to/")
-        XCTAssertEqual("Ma\u{308}dchen".commonPrefix(with: "Mädchenschule"), "Ma\u{308}dchen")
         XCTAssertEqual("this".commonPrefix(with: "THAT", options: [.caseInsensitive]), "th")
-        XCTAssertEqual("this".commonPrefix(with: "THAT", options: [.caseInsensitive, .literal]), "th")
-        XCTAssertEqual("Ma\u{308}dchen".commonPrefix(with: "Mädchenschule", options: [.literal]), "M")
+
+        // Both forms of ä, a\u{308} decomposed and \u{E4} precomposed, should match without .literal and not match when .literal is used
+        XCTAssertEqual("Ma\u{308}dchen".commonPrefix(with: "M\u{E4}dchenschule"), "Ma\u{308}dchen")
+        XCTAssertEqual("Ma\u{308}dchen".commonPrefix(with: "M\u{E4}dchenschule", options: [.literal]), "M")
+        XCTAssertEqual("m\u{E4}dchen".commonPrefix(with: "M\u{E4}dchenschule", options: [.caseInsensitive, .literal]), "mädchen")
+        XCTAssertEqual("ma\u{308}dchen".commonPrefix(with: "M\u{E4}dchenschule", options: [.caseInsensitive, .literal]), "m")
     }
 }