debuginfo: create_scope_map.rs: change discriminators map capacity to…

[edrf0]

… match mir.source_scopes length

Aligns discriminators FxHashMap capacity with other scope-related data structures (vars, instantiated) using mir.source_scopes.len().

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[Zalathar]

I don't see any argument or explanation for why discriminators should have an initial capacity of mir.source_scopes.len().

What reason is there to think that that's an appropriate capacity?