[PR 56419/c++]: C++ front end silently drops transactions

[Aldy Hernandez <aldyh at redhat dot com>]

In the following snippet, the C++ front-end drops the transaction 
altogether:

+int x = 0;
+int inc_func(int i) {
+     for (int j = 0; j < i; ++j)
+     {
+         __transaction_atomic { x+=1; }
+     }
+     return 0;

This was caused by http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=186546

The problem here is that genericize_cp_loop() calls 
append_to_statement_list() to add the TRANSACTION_EXPR, but in this 
case, TREE_SIDE_EFFECTS is not set, so it is silently ignored.

Frankly, I don't understand finish_transaction_stmt(), and why it sets 
TREE_SIDE_EFFECTS only if the [noexcept] clause is set.  I'm C++ 
ignorant, but I would've thought the opposite to be true.

Anyways, I've fixed the problem by setting TREE_SIDE_EFFECTS if the 
transaction body has side effects.  Perhaps we should do this for 
build_transaction_expr() as well?

What do y'all prefer?

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