Use bound_minimal_symbol in var_msym_value_operation
This changes var_msym_value_operation to use a bound_minimal_symbol
rather than separate minsym and objfile parameters. The main benefit
of this is removing the possibly-confusing check_objfile overload for
a plain minimal symbol.
gdb/ChangeLog
2021-03-08 Tom Tromey <tom@tromey.com>