c++/coros: do not assume coros don't nest [PR113457]

c++/coros: do not assume coros don't nest [PR113457]

In the testcase presented in the PR, during template expansion, an
tsubst of an operand causes a lambda coroutine to be processed, causing
it to get an initial suspend and final suspend.  The code for assigning
awaitable var names (get_awaitable_var) assumed that the sequence Is ->
Is -> Fs -> Fs is impossible (i.e. that one could only 'open' one
coroutine before closing it at a time), and reset the counter used for
unique numbering each time a final suspend occured.  This assumption is
false in a few cases, usually when lambdas are involved.

Instead of storing this counter in a static-storage variable, we can
store it in coroutine_info.  This struct is local to each function, so
we don't need to worry about "cross-contamination" nor resetting.

	PR c++/113457

gcc/cp/ChangeLog:

	* coroutines.cc (struct coroutine_info): Add integer field
	awaitable_number.  This is a counter used for assigning unique
	names to awaitable temporaries.
	(get_awaitable_var): Use awaitable_number from coroutine_info
	instead of the static int awn.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/pr113457-1.C: New test.
	* g++.dg/coroutines/pr113457.C: New test.