Garbage collection bugs

[Arseny Solokha]

> First problem:
> 
> The g++.dg/pr85039-2.C tests (I've looked in detail at -std=c++98, but
> -std=c++11 and -std=c++14 appear to follow the same pattern) see gcc
> garbage-collecting a live vector.  A subsequent access to the vector with
> vec_quick_push causes a segmentation fault, as m_vecpfx.m_num is 0xa5a5a5a5 .
> The vec data is also being freed / poisoned. The vector in question is an
> auto-variable of cp_parser_parenthesized_expression_list, which is declared as:
> vec<tree, va_gc> *expression_list;

It looks like PR88180 to me.