[llvm] [FileCheck] Add split view diff option for FileCheck (PR #189269)

[Henrik G. Olsson via llvm-commits]

================
@@ -2775,25 +2809,58 @@ static void renderDiff(unsigned ExpectedLineNo, unsigned ActualLineNo,
 
   // Before Context
   if (!Ctx.LineBefore.empty()) {
-    OS << " " << Ctx.LineBefore << "\n";
+    if (Mode == Split)
+      OS << "  " << GetView(Ctx.LineBefore) << Sep << GetView(Ctx.LineBefore)
+         << "\n";
+    else
+      OS << " " << Ctx.LineBefore << "\n";
   }
 
   // Mismatch
-  OS.changeColor(raw_ostream::RED);
-  OS << "-" << ExpectedLine << "\n";
+  if (Mode == Split) {
+    OS << "  ";
 
-  OS.changeColor(raw_ostream::GREEN);
-  OS << "+" << ActualLine.ltrim() << "\n";
-  OS.resetColor();
+    OS.changeColor(raw_ostream::RED);
+    OS << GetView(ExpectedLine);
+    OS.resetColor();
+
+    // Use a bold yellow '!' to signify that the text might match,
+    // but it was found on the wrong line.
+    bool IsMoved = (ExpectedLineNo != ActualLineNo);
+
+    if (IsMoved) {
+      OS.changeColor(raw_ostream::YELLOW, true);
+      OS << " ! ";
+    } else {
+      OS << " | ";
----------------
hnrklssn wrote:

```suggestion
      OS << Sep;
```

https://github.com/llvm/llvm-project/pull/189269