c++: Fix ICE due to shared BLOCK node in coroutine generation [PR103328]

When finishing a function that is a coroutine, the function is
transformed into a "ramp" function, and the original user-provided
function body gets moved into a newly created "actor" function.

In this case `current_function_decl` points to the ramp function,
but `current_binding_level->blocks` would still point to the
scope block of the user-provided function body in the actor function,
so when the ramp function was finished during `poplevel()` in decl.cc,
we could end up with that block being reused as the `DECL_INITIAL()` of
the ramp function:

   subblocks = functionbody >= 0 ? current_binding_level->blocks : 0;
   // [...]
   DECL_INITIAL (current_function_decl) = block ? block : subblocks;

This block would then be independently modified by subsequent passes
touching either the ramp or the actor function, potentially causing
an ICE depending on the order and function of these passes.

gcc/cp/ChangeLog:

	PR c++/103328
	* coroutines.cc (morph_fn_to_coro): Reset
	current_binding_level->blocks.

gcc/testsuite/ChangeLog:

	PR c++/103328
	* g++.dg/coroutines/pr103328.C: New test.

Co-Authored-By: Iain Sandoe <iain@sandoe.co.uk>

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