Support referring to Self in bitflags constants

[jonas-schievink]

The initial bitflags support in #528 had a restriction where referring to the Self type in a bitflags constant would result in a compile error.

Turns out that's entirely unnecessary and I wasn't quite awake when I wrote that code. A minor modification of the generated code avoids this somewhat annoying restriction. Instead of copying the expression verbatim, we can just refer to the associated constant created by the bitflags crate, and extract the raw bits.

[Urhengulas]

bors r+

[bors]

Build succeeded:

• ci