Fix race in background index-cache writing

Tom de Vries pointed out a bug in the index-cache background writer --
sometimes it will fail.  He also noted that it fails when the number
of worker threads is set to zero.  These turn out to be the same
problem -- the cache can't be written to until the per-BFD's
"index_table" member is set.

This patch avoids the race by rearranging the code slightly, to ensure
the cache cannot possibly be written before the member is set.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30261

gdb/dwarf2/cooked-index.h

@@ -366,7 +366,7 @@ public:
      object.  */
   using vec_type = std::vector<std::unique_ptr<cooked_index_shard>>;
 
-  cooked_index (vec_type &&vec, dwarf2_per_bfd *per_bfd);
+  explicit cooked_index (vec_type &&vec);
   ~cooked_index () override;
   DISABLE_COPY_AND_ASSIGN (cooked_index);
 
@@ -429,6 +429,9 @@ public:
     m_write_future.wait ();
   }
 
+  /* Start writing to the index cache, if the user asked for this.  */
+  void start_writing_index (dwarf2_per_bfd *per_bfd);
+
 private:
 
   /* Maybe write the index to the index cache.  */