Add IRGen support for error results from functions.

As part of this, re-arrange the argument order so that
generic arguments come before the context, which comes
before the error result.  Be more consistent about always
adding a context parameter on thick functions, even
when it's unused.  Pull out the witness-method Self
argument so that it appears last after the error
argument.

Swift SVN r26667

lib/IRGen/IRGenFunction.cpp

@@ -35,7 +35,7 @@ IRGenFunction::IRGenFunction(IRGenModule &IGM,
                              SILDebugScope *DbgScope,
                              Optional<SILLocation> DbgLoc)
   : IGM(IGM), Builder(IGM.getLLVMContext()),
-    CurFn(Fn), ContextPtr(nullptr), DbgScope(DbgScope)
+    CurFn(Fn),  DbgScope(DbgScope)
   {
 
   // Make sure the instructions in this function are attached its debug scope.