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
f795c529a8