gdb: add noexcept to relevant methods of class ref_ptr

This patch aims at improving code readability and maintainability
by adding the 'noexcept' attribute to some constructors and
methods of class 'ref_ptr' to make clear that no exception is
supposed to be raised from them.
As an additional benefit, the compiler doesn't need to generate
the stack unwinding code for those constructors and methods.

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