Merge pull request #4986 from jmdavis/string_trivial

Make std.string.StringException use basicExceptionCtors.

std/string.d

@@ -209,20 +209,10 @@ public import std.array : join, replace, replaceInPlace, split, empty;
   +/
 class StringException : Exception
 {
-    /++
-        Params:
-            msg  = The message for the exception.
-            file = The file where the exception occurred.
-            line = The line number where the exception occurred.
-            next = The previous exception in the chain of exceptions, if any.
-      +/
-    this(string msg,
-         string file = __FILE__,
-         size_t line = __LINE__,
-         Throwable next = null) @safe pure nothrow
-    {
-        super(msg, file, line, next);
-    }
+    import std.exception : basicExceptionCtors;
+
+    ///
+    mixin basicExceptionCtors;
 }