[spec] First enum member is converted from 0 to base type

[ntrel]

Fix Issue 18578 - First enum value assigned 0 instead of EnumBaseType.init.

Add an example also showing overflow.

@WalterBright suggested fixing the spec rather than breaking existing code:
dlang/dmd#7996 (comment)

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	18578	normal	First enum value assigned 0 instead of EnumBaseType.init