lang: Fix cpi feature instructions not accounting for discriminator overrides

[acheroncrypto]

Problem

CPI instruction discriminators do not take into account the potential discriminator overrides from the #[interface] attribute (#2728) or the new custom discriminator support via #[instruction(discriminator = <VALUE>)] (#3137):

anchor/lang/syn/src/codegen/program/cpi.rs

Line 20 in 8ec7ec8

let discriminator = gen_discriminator(SIGHASH_GLOBAL_NAMESPACE, name);

Summary of changes

• Fix cpi feature instructions not accounting for discriminator overrides
• Perform a small refactor to reduce cloning and code duplication of related code

[vercel]

@acheroncrypto is attempting to deploy a commit to the coral-xyz Team on Vercel.

A member of the Team first needs to authorize it.