Use overload APIs that take char instead of string when it's possible (PowerShell#18179)

Author: iSazonov

src/Microsoft.PowerShell.Commands.Management/commands/management/RegisterWMIEventCommand.cs

@@ -91,7 +91,7 @@ private string GetScopeString(string computer, string namespaceParameter)
         {
             StringBuilder returnValue = new StringBuilder("\\\\");
             returnValue.Append(computer);
-            returnValue.Append("\\");
+            returnValue.Append('\\');
             returnValue.Append(namespaceParameter);
             return returnValue.ToString();
         }

src/Microsoft.PowerShell.Commands.Management/commands/management/WMIHelper.cs

@@ -1042,7 +1042,7 @@ internal static string GetScopeString(string computer, string namespaceParameter
         {
             StringBuilder returnValue = new StringBuilder("\\\\");
             returnValue.Append(computer);
-            returnValue.Append("\\");
+            returnValue.Append('\\');
             returnValue.Append(namespaceParameter);
             return returnValue.ToString();
         }
@@ -1659,7 +1659,7 @@ private string ConstructLocation()
             foreach (PSWmiChildJob job in ChildJobs)
             {
                 location.Append(job.Location);
-                location.Append(",");
+                location.Append(',');
             }
 
             location.Remove(location.Length - 1, 1);

src/Microsoft.PowerShell.Security/security/CertificateProvider.cs

@@ -1568,7 +1568,7 @@ private static string NormalizePath(string path)
 
                 string[] elts = GetPathElements(path);
 
-                path = string.Join("\\", elts);
+                path = string.Join('\\', elts);
             }
 
             return path;

src/System.Management.Automation/DscSupport/CimDSCParser.cs

@@ -992,7 +992,7 @@ public static List<CimClass> ImportClasses(string path, Tuple<string, Version> m
                         // allow sharing of nested objects.
                         if (!IsSameNestedObject(cimClass, c))
                         {
-                            var files = string.Join(",", GetFileDefiningClass(className));
+                            var files = string.Join(',', GetFileDefiningClass(className));
                             PSInvalidOperationException e = PSTraceSource.NewInvalidOperationException(
                                 ParserStrings.DuplicateCimClassDefinition, className, path, files);
 

src/System.Management.Automation/engine/CommandDiscovery.cs

@@ -454,7 +454,7 @@ internal static void VerifyPSEdition(ExternalScriptInfo scriptInfo)
                 //
                 if (isRequiresPSEditionSpecified && !isCurrentEditionListed)
                 {
-                    var specifiedEditionsString = string.Join(",", scriptInfo.RequiresPSEditions);
+                    var specifiedEditionsString = string.Join(',', scriptInfo.RequiresPSEditions);
                     var message = StringUtil.Format(DiscoveryExceptions.RequiresPSEditionNotCompatible,
                         scriptInfo.Name,
                         specifiedEditionsString,

test/tools/WebListener/Controllers/LinkController.cs

@@ -87,7 +87,7 @@ public JsonResult Index()
             }
             else
             {
-                linkHeader = string.Join(",", linkList);
+                linkHeader = string.Join(',', linkList);
             }
 
             Response.Headers.Add("Link", linkHeader);