coroutines: Fix cases where proxy variables are used [PR94879]

There are several places where the handling of a variable
declaration depends on whether it corresponds to a compiler
temporary, or to some other entity.  We were testing that var
decls were artificial in determining this.  However, proxy vars
are also artificial so that this is not sufficient.  The solution
is to exclude variables with a DECL_VALUE_EXPR as well, since
the value variable will not be a temporary.

gcc/cp/ChangeLog:

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

	PR c++/94879
	* coroutines.cc (build_co_await): Account for variables
	with DECL_VALUE_EXPRs.
	(captures_temporary): Likewise.
	(register_awaits): Likewise.

gcc/testsuite/ChangeLog:

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

	PR c++/94879
	* g++.dg/coroutines/pr94879-folly-1.C: New test.