[Windows] Remove extra newlines from diagnostic output.

We are outputting lots of extra newlines on Windows, which makes it
hard to read the diagnostics.

rdar://148520063

Author: al45tair

Diff for: Sources/SwiftDiagnostics/DiagnosticsFormatter.swift

@@ -203,6 +203,19 @@ public struct DiagnosticsFormatter {
     return resultSourceString
   }
 
+  /// Strip trailing newlines
+  ///
+  /// - Parameters:
+  ///   - s: string from which to strip trailing newlines.
+  func stripNewlines(
+    _ s: String
+  ) -> String {
+    if let lastNewline = s.lastIndex(where: { $0.isNewline }) {
+      return String(s.prefix(upTo: lastNewline))
+    }
+    return s
+  }
+
   /// Print given diagnostics for a given syntax tree on the command line
   ///
   /// - Parameters:
@@ -232,8 +245,12 @@ public struct DiagnosticsFormatter {
         return nil
       }.joined()
 
+      // Strip trailing newlines
+      let strippedLine = stripNewlines(sourceLine)
+      let strippedSuffix = stripNewlines(suffixText)
+
       annotatedSourceLines.append(
-        AnnotatedSourceLine(diagnostics: diagsForLine, sourceString: sourceLine, suffixText: suffixText)
+        AnnotatedSourceLine(diagnostics: diagsForLine, sourceString: strippedLine, suffixText: strippedSuffix)
       )
     }
 
@@ -293,10 +310,7 @@ public struct DiagnosticsFormatter {
         )
       )
 
-      // If the line did not end with \n (e.g. the last line), append it manually
-      if annotatedSource.last != "\n" {
-        annotatedSource.append("\n")
-      }
+      annotatedSource.append("\n")
 
       let columnsWithDiagnostics = Set(
         annotatedLine.diagnostics.map {
@@ -331,8 +345,8 @@ public struct DiagnosticsFormatter {
       }
 
       // Add suffix text.
-      annotatedSource.append(annotatedLine.suffixText)
-      if annotatedSource.last != "\n" {
+      if !annotatedLine.suffixText.isEmpty {
+        annotatedSource.append(annotatedLine.suffixText)
         annotatedSource.append("\n")
       }
     }