RFA (hash-map): PATCH to support GTY((cache)) with hash_map

[Jason Merrill <jason at redhat dot com>]

The hash_map interface is a lot more convenient than that of
hash_table for cases where it makes sense, but there hasn't been a way
to get the ggc_cache_remove behavior with a hash_map.  In other words,
not marking elements during the initial ggc marking phase, but maybe
marking them during the clear_caches phase based on keep_cache_entry.

This patch implements that by:

Adding a ggc_maybe_mx member function to ggc_remove, and overriding
that instead of ggc_mx in ggc_cache_remove.
Calling H::ggc_maybe_mx instead of H::ggc_mx in gt_ggc_mx (hash_table *).
Calling H::ggc_mx in gt_cleare_caches (hash_table *) rather than
relying on an extern declaration of a plain function that cannot be
declared for hash_map::hash_entry.
Adding ggc_maybe_mx and keep_cache_entry to hash_map::hash_entry.
Adding gt_cleare_cache for hash_map.
Adding a boolean constant to the hash-map traits indicating whether we
want the cache behavior above.

I then define a typedef tree_cache_map to use this functionality, and
use it in a few places in the C++ front end.

Tested x86_64-pc-linux-gnu, OK for trunk?

Attachment: cache-map.diff
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