Add frontend flags for developers to easily control copy propagation:

-enable-copy-propagation: enables whatever form of copy propagation
 the current pipeline runs (mandatory-copy-propagation at -Onone,
 regular copy-propation at -O).

-disable-copy-propagation: similarly disables any form of copy
 propagation in the current pipelien.

lib/SILOptimizer/Transforms/CopyPropagation.cpp

@@ -136,11 +136,9 @@ void CopyPropagation::run() {
 }
 
 SILTransform *swift::createMandatoryCopyPropagation() {
-  return new CopyPropagation(/*pruneDebug*/ true, /*unownedRemnant*/ true,
-                             /*canonicalizeAll*/ true);
+  return new CopyPropagation(/*pruneDebug*/ true, /*canonicalizeAll*/ true);
 }
 
 SILTransform *swift::createCopyPropagation() {
-  return new CopyPropagation(/*pruneDebug*/ true, /*unownedRemnant*/ false,
-                             /*canonicalizeAll*/ false);
+  return new CopyPropagation(/*pruneDebug*/ true, /*canonicalizeAll*/ false);
 }