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

Since GDB now requires C++17, we don't need the internally maintained
gdb::optional implementation.  This patch does the following replacing:
  - gdb::optional -> 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
Approved-By: Tom Tromey <tom@tromey.com>
Approved-By: Pedro Alves <pedro@palves.net>

gdb/linux-nat.h

@@ -103,7 +103,7 @@ public:
 		   int flags, int mode, int warn_if_slow,
 		   fileio_error *target_errno) override;
 
-  gdb::optional<std::string>
+  std::optional<std::string>
     fileio_readlink (struct inferior *inf,
 		     const char *filename,
 		     fileio_error *target_errno) override;