Fix heap-use-after-free in index-cached with --disable-threading

If threads are disabled, either by --disable-threading explicitely, or by
missing std::thread support, you get the following ASAN error when
loading symbols:

==7310==ERROR: AddressSanitizer: heap-use-after-free on address 0x614000002128 at pc 0x00000098794a bp 0x7ffe37e6af70 sp 0x7ffe37e6af68
READ of size 1 at 0x614000002128 thread T0
    #0 0x987949 in index_cache_store_context::store() const ../../gdb/dwarf2/index-cache.c:163
    #1 0x943467 in cooked_index_worker::write_to_cache(cooked_index const*, deferred_warnings*) const ../../gdb/dwarf2/cooked-index.c:601
    #2 0x1705e39 in std::function<void ()>::operator()() const /gcc/9/include/c++/9.2.0/bits/std_function.h:690
    #3 0x1705e39 in gdb::task_group::impl::~impl() ../../gdbsupport/task-group.cc:38

0x614000002128 is located 232 bytes inside of 408-byte region [0x614000002040,0x6140000021d8)
freed by thread T0 here:
    #0 0x7fd75ccf8ea5 in operator delete(void*, unsigned long) ../../.././libsanitizer/asan/asan_new_delete.cc:177
    #1 0x9462e5 in cooked_index::index_for_writing() ../../gdb/dwarf2/cooked-index.h:689
    #2 0x9462e5 in operator() ../../gdb/dwarf2/cooked-index.c:657
    #3 0x9462e5 in _M_invoke /gcc/9/include/c++/9.2.0/bits/std_function.h:300

It's happening because cooked_index_worker::wait always returns true in
this case, which tells cooked_index::wait it can delete the m_state
cooked_index_worker member, but cooked_index_worker::write_to_cache tries
to access it immediately afterwards.

Fixed by making cooked_index_worker::wait only return true if desired_state
is CACHE_DONE, same as if threading was enabled, so m_state will not be
prematurely deleted.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31694


Approved-By: Tom Tromey <tom@tromey.com>