Introduce and use attribute::unsigned_constant

Introduce and use attribute::unsigned_constant

This introduces a new 'unsigned_constant' method on attribute.  This
method can be used to get the value as an unsigned number.  Unsigned
scalar forms are handled, and signed scalar forms are handled as well
provided that the value is non-negative.

Several spots in the reader that expect small DWARF-defined constants
are updated to use this new method.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32680
Approved-By: Simon Marchi <simon.marchi@efficios.com>