gdb: modernize get_frame_pc_if_available

gdb: modernize get_frame_pc_if_available

The convenience function get_frame_pc_if_available would take a pointer
to a variable that should be set if available, and would return a
boolean indicating whether that action was successful or not.

Now that GDB supports C++17 features, this indirection of a pointer and
returning boolean is unnecessary, since the function can return an
optional, and code that calls it can check if the optional contains a
value.

This commit makes that modernization. It should have no visible
effects.

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