[CSApply] NFC: Simplify coerceSelfArgumentToType

`coerceToType` can to handle l-value -> inout correctly
and report better information if coercion fails.

(cherry picked from commit c225db4)

Author: xedin

lib/Sema/CSApply.cpp

@@ -7980,26 +7980,7 @@ ExprRewriter::coerceSelfArgumentToType(Expr *expr,
                                        Type baseTy, ValueDecl *member,
                                        ConstraintLocatorBuilder locator) {
   Type toType = adjustSelfTypeForMember(expr, baseTy, member, dc);
-
-  // If our expression already has the right type, we're done.
-  Type fromType = cs.getType(expr);
-  if (fromType->isEqual(toType))
-    return expr;
-
-  // If we're coercing to an rvalue type, just do it.
-  auto toInOutTy = toType->getAs<InOutType>();
-  if (!toInOutTy)
-    return coerceToType(expr, toType, locator);
-
-  assert(fromType->is<LValueType>() && "Can only convert lvalues to inout");
-
-  auto &ctx = cs.getASTContext();
-
-  // Use InOutExpr to convert it to an explicit inout argument for the
-  // receiver.
-  return cs.cacheType(new (ctx) InOutExpr(expr->getStartLoc(), expr, 
-                                          toInOutTy->getInOutObjectType(),
-                                          /*isImplicit*/ true));
+  return coerceToType(expr, toType, locator);
 }
 
 Expr *ExprRewriter::convertLiteralInPlace(