Update copyright year range in all GDB files.

This commit applies all changes made after running the gdb/copyright.py
script.

Note that one file was flagged by the script, due to an invalid
copyright header
(gdb/unittests/basic_string_view/element_access/char/empty.cc).
As the file was copied from GCC's libstdc++-v3 testsuite, this commit
leaves this file untouched for the time being; a patch to fix the header
was sent to gcc-patches first.

gdb/ChangeLog:

	Update copyright year range in all GDB files.