Add handling for unqualified Ada operators in linespecs

Add handling for unqualified Ada operators in linespecs

This patch enhances the linespec parser to recognize unqualified
operator names in linespecs. This allows the user to insert a breakpoint
on operator "+" as follow, for instance:

        (gdb) break "+"

Previously, it was possible to insert such a breakpoint, but one
had to fully qualify the function name. For instance:

        (gdb) break ops."+"

gdb/ChangeLog:

        * linespec.c (locate_first_half): Add handling of Ada operators
        when the current language is Ada.