Fix regcache leak, and avoid possible regcache access after detach.

Fix regcache leak, and avoid possible regcache access after detach.

Valgrind reports leaks like the below in various tests,
e.g. gdb.threads/attach-slow-waitpid.exp, gdb.ada/task_switch_in_core.exp, ...

Fix the leak by clearing the regcache when detaching from an inferior.
Note that these leaks are 'created' when GDB exits,
when the regcache::current_regcache is destroyed : the elements
of the forward_list are pointers, and the 'pointed to' memory is not
deleted by the forward_list destructor.

Nevertheless, fixing this leak is good as it makes a bunch of
tests 'leak clean'.

Also, it seems strange to keep a register cache for a process from
which GDB detached : it is not clear if this cache is still valid
after detach.  And effectively, when clearing only the regcache,
(and not the frame cache), then the frame cache was still 'pointing'
at this regcache and was used when switching to the child process
in the test gdb.threads/watchpoint-fork.exp, which seems strange.

So, we solve the leak and avoid possible accesses to the regcache
and frame cache of the detached inferior, by clearing both the
regcache and the frame cache.

Tested on debian/amd64, natively, under Valgrind,
and with make check RUNTESTFLAGS="--target_board=native-gdbserver".

==27679== VALGRIND_GDB_ERROR_BEGIN
==27679== 1,123 (72 direct, 1,051 indirect) bytes in 1 blocks are definitely lost in loss record 2,942 of 3,400
==27679==    at 0x4C2C4CC: operator new(unsigned long) (vg_replace_malloc.c:344)
==27679==    by 0x5CDF71: get_thread_arch_aspace_regcache(ptid_t, gdbarch*, address_space*) (regcache.c:330)
==27679==    by 0x5CE12A: get_thread_regcache (regcache.c:366)
==27679==    by 0x5CE12A: get_current_regcache() (regcache.c:372)
==27679==    by 0x4FF63D: post_create_inferior(target_ops*, int) (infcmd.c:452)
==27679==    by 0x43AF62: core_target_open(char const*, int) (corelow.c:458)
==27679==    by 0x408B68: cmd_func(cmd_list_element*, char const*, int) (cli-decode.c:1892)
...

gdb/ChangeLog
2019-02-27  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* target.c (target_detach): Clear the regcache and the
	frame cache.