c++: Fix explicit instantiation of const variable templates after earlier...

c++: Fix explicit instantiation of const variable templates after earlier implicit instantation [PR113976]

Already previously instantiated const variable templates had
cp_apply_type_quals_to_decl called when they were instantiated,
but if they need runtime initialization, their TREE_READONLY flag
has been subsequently cleared.
Explicit variable template instantiation calls grokdeclarator which
calls cp_apply_type_quals_to_decl on them again, setting TREE_READONLY
flag again, but nothing clears it afterwards, so we emit such
instantiations into rodata sections and segfault when the dynamic
initialization attempts to initialize them.

The following patch fixes that by not calling cp_apply_type_quals_to_decl
on already instantiated variable declarations.

2024-02-28  Jakub Jelinek  <jakub@redhat.com>
	    Patrick Palka  <ppalka@redhat.com>

	PR c++/113976
	* decl.cc (grokdeclarator): Don't call cp_apply_type_quals_to_decl
	on DECL_TEMPLATE_INSTANTIATED VAR_DECLs.

	* g++.dg/cpp1y/var-templ87.C: New test.