[PATCH v2 00/12] Work toward making psymtabs reusable

[Tom Tromey]

This is version 2 of my series to improve psymtabs a bit.  In
particular this moves most psymtab allocation to a special obstack, to
make it simpler to reason about.  Mostly this is preparation for
(hopeful) future patches to make psymtabs independent of the objfile.

Version 1 was here:

https://sourceware.org/ml/gdb-patches/2018-05/msg00276.html

... with reviews mostly happening in July.

This addresses all the review comments.  I've also dropped the patch
that removed the psymtab->symtab backlink and updated various comments
accordingly.  That patch caused a small performance regression, and in
the absence of supporting comments it seemed simpler to remove it.  I
may make another push for it later, when it's more needed.

Regression tested on the buildbot.

Tom