Auto merge of #106283 - JulianKnodt:enum_err, r=cjgillot

Add help diag. for `const = Enum` missing braces around `Enum`

Previously it was not clear why this errored or if it was even supported, as there was no diagnostic that suggested wrapping it in braces.

Thus, add a simple diagnostic that suggests wrapping enum variants in braces.

Fixes #105927

Author: bors