gdb: convert type instance flags to bitfields

gdb: convert type instance flags to bitfields

Convert the instance flags of a type to a struct with bitfields.  This
helps avoid bitwise operations and instead refer to the fields by
name.  In particular, Harvard address space information (i.e. code
space and data space) and address class information become enum values
instead of being handled by seemingly independent bits.

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