varasm: Fix ICE with -fsyntax-only [PR99035]

varasm: Fix ICE with -fsyntax-only [PR99035]

My FE change from 2 years ago uses TREE_ASM_WRITTEN in -fsyntax-only
mode more aggressively to avoid "expanding" functions multiple times.
With -fsyntax-only nothing is really expanded, so I think it is acceptable
to adjust the assert and allow declare_weak at any time, with -fsyntax-only
we know it is during parsing only anyway.

2021-02-10  Jakub Jelinek  <jakub@redhat.com>

	PR c++/99035
	* varasm.c (declare_weak): For -fsyntax-only, allow even
	TREE_ASM_WRITTEN function decls.

	* g++.dg/ext/weak6.C: New test.

(cherry picked from commit a964f494cd5a90f631b8c0c01777a9899e0351ce)