Use "default" color for diagnostic highlights

Author: DougGregor

Sources/SwiftDiagnostics/DiagnosticsFormatter.swift

@@ -372,6 +372,7 @@ struct ANSIAnnotation {
     case magenta = 35
     case cyan = 36
     case white = 37
+    case `default` = 39
   }
 
   enum Trait: UInt8 {
@@ -414,6 +415,6 @@ struct ANSIAnnotation {
 
   /// Annotation used for highlighting source text.
   static var sourceHighlight: ANSIAnnotation {
-    ANSIAnnotation(color: .white, trait: .underline)
+    ANSIAnnotation(color: .default, trait: .underline)
   }
 }

Tests/SwiftDiagnosticsTest/DiagnosticsFormatterTests.swift

@@ -127,7 +127,7 @@ final class DiagnosticsFormatterTests: XCTestCase {
       """
 
     let expectedOutput = """
-      \u{001B}[0;36m1 │\u{001B}[0;0m for \u{001B}[4;37m(i\u{001B}[0;0m \u{001B}[4;37m= 0; i != 10; i += 1)\u{001B}[0;0m { }
+      \u{001B}[0;36m1 │\u{001B}[0;0m for \u{001B}[4;39m(i\u{001B}[0;0m \u{001B}[4;39m= 0; i != 10; i += 1)\u{001B}[0;0m { }
         \u{001B}[0;36m∣\u{001B}[0;0m │      ╰─ \u{001B}[1;31merror: expected ')' to end tuple pattern\u{001B}[0;0m
         \u{001B}[0;36m∣\u{001B}[0;0m ╰─ \u{001B}[1;31merror: C-style for statement has been removed in Swift 3\u{001B}[0;0m