Fix C# wrappers FxCop warning CA2002 in SWIGPendingException

Fixes two warnings in each wrapper:

warning  : CA2002 : Microsoft.Reliability : 'examplePINVOKE.SWIGPendingException.Retrieve()' locks on a reference of type 'Type'. Replace this with a lock against an object with strong-identity.

warning  : CA2002 : Microsoft.Reliability : 'examplePINVOKE.SWIGPendingException.Set(Exception)' locks on a reference of type 'Type'. Replace this with a lock against an object with strong-identity.

Use lock statement advice not to use typeof for locks, see
https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/lock-statement

Author: wsfulton

CHANGES.current

@@ -7,6 +7,10 @@ the issue number to the end of the URL: https://github.com/swig/swig/issues/
 Version 4.0.0 (in progress)
 ===========================
 
+2019-04-09: wsfulton
+            [C#] Fix FxCop warning CA2002 in SWIGPendingException - a lock on a reference of
+            type 'Type'.
+
 2019-03-30: wsfulton
             [Java, D] Add the parameters typemap attribute to the javadestruct,
             javadestruct_derived, ddispose, ddispose_derived typemaps to mirror enhanced

Lib/csharp/csharphead.swg

@@ -244,6 +244,7 @@ SWIGEXPORT void SWIGSTDCALL SWIGRegisterExceptionArgumentCallbacks_$module(
     [global::System.ThreadStatic]
     private static global::System.Exception pendingException = null;
     private static int numExceptionsPending = 0;
+    private static global::System.Object exceptionsLock = null;
 
     public static bool Pending {
       get {
@@ -259,7 +260,7 @@ SWIGEXPORT void SWIGSTDCALL SWIGRegisterExceptionArgumentCallbacks_$module(
       if (pendingException != null)
         throw new global::System.ApplicationException("FATAL: An earlier pending exception from unmanaged code was missed and thus not thrown (" + pendingException.ToString() + ")", e);
       pendingException = e;
-      lock(typeof($imclassname)) {
+      lock(exceptionsLock) {
         numExceptionsPending++;
       }
     }
@@ -270,13 +271,17 @@ SWIGEXPORT void SWIGSTDCALL SWIGRegisterExceptionArgumentCallbacks_$module(
         if (pendingException != null) {
           e = pendingException;
           pendingException = null;
-          lock(typeof($imclassname)) {
+          lock(exceptionsLock) {
             numExceptionsPending--;
           }
         }
       }
       return e;
     }
+
+    static SWIGPendingException() {
+      exceptionsLock = new global::System.Object();
+    }
   }
 %}
 #endif // SWIG_CSHARP_NO_EXCEPTION_HELPER