Use / in paths in the auto-generated files' header (microsoft#24883)

Author: sandersn

scripts/processDiagnosticMessages.ts

@@ -60,7 +60,7 @@ function checkForUniqueCodes(diagnosticTable: InputDiagnosticMessageTable) {
 function buildInfoFileOutput(messageTable: InputDiagnosticMessageTable, inputFilePathRel: string, thisFilePathRel: string): string {
     let result =
         "// <auto-generated />\r\n" +
-        "// generated from '" + inputFilePathRel + "' by '" + thisFilePathRel + "'\r\n" +
+        "// generated from '" + inputFilePathRel + "' by '" + thisFilePathRel.replace(/\\/g, '/') + "'\r\n" +
         "/* @internal */\r\n" +
         "namespace ts {\r\n" +
         "    function diag(code: number, category: DiagnosticCategory, key: string, message: string, reportsUnnecessary?: {}): DiagnosticMessage {\r\n" +
@@ -119,4 +119,4 @@ function convertPropertyName(origName: string): string {
     return result;
 }
 
-main();
+main();