[trans-mem] Fix instantiation of transaction expressions.

[Torvald Riegel <triegel at redhat dot com>]

I stumbled upon this when working on noexcept. When txn expressions were
processed in tsubst_expr(), the previous code incorrectly created empty
transaction statements because no statements got added to the statement
list used by (begin|finish)_transaction_stmt(). Also,
"return __transaction_atomic (x+1);" incorrectly returned an error that
this wouldn't return a value.
With the patch, if RECUR(...) returns a value, then we assume that this
is an expression and create a txn expression instead.

No regressions for the TM tests, running a bootstrap currently.

OK for branch?

Attachment: patch3
Description: Text document