gdb: Replace gdb::optional with std::optional

Since GDB now requires a C++17, we don't need the internally maintained
gdb::optional implementation.  This patch does the following replacing:
  - gdb::optonal -> std::optional
  - gdb::in_place -> std::in_place
  - #include "gdbsupport/gdb_optional.h" -> #include <optional>

This change has mostly been done automatically.  One exception is
gdbsupport/thread-pool which did not use the gdb:: prefix as it already
lives in the gdb namespace.

Change-Id: I19a92fa03e89637bab136c72e34fd351524f65e9

gdb/cli/cli-decode.h

@@ -233,7 +233,7 @@ struct cmd_list_element
   void (*destroyer) (struct cmd_list_element *self, void *context) = nullptr;
 
   /* Setting affected by "set" and "show".  Not used if type is not_set_cmd.  */
-  gdb::optional<setting> var;
+  std::optional<setting> var;
 
   /* Pointer to NULL terminated list of enumerated values (like
      argv).  */