coroutines: Fix handling of artificial vars [PR94886]

coroutines: Fix handling of artificial vars [PR94886]

The testcase ICEs because the range-based for generates three
artificial variables that need to be allocated to the coroutine
frame but, when walking the BIND_EXR that contains these, the
DECL_INITIAL for one of them refers to an entry appearing later,
which means that the frame entry hasn't been allocated when that
INITIAL is walked.

The solution is to defer walking the DECL_INITIAL/SIZE etc. until
all the BIND_EXPR vars have been processed.

gcc/cp/ChangeLog:

2020-04-30  Iain Sandoe  <iain@sandoe.co.uk>

	PR c++/94886
	* coroutines.cc (transform_local_var_uses): Defer walking
	the DECL_INITIALs of BIND_EXPR vars until all the frame
	allocations have been made.

gcc/testsuite/ChangeLog:

2020-04-30  Iain Sandoe  <iain@sandoe.co.uk>

	PR c++/94886
	* g++.dg/coroutines/pr94886-folly-3.C: New test.