Support field access on variables of type Enum

[sbomer]

Follow-up to #105351

This adds support for the case when a variable is of type Enum and there are no generics involved, so the following no longer produces trim warnings:

static void M(Enum v) {
    v.GetType().GetFields();
}

Fixes #105506

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[sbomer]

/ba-g "unrelated timeout in Workloads-NoFingerprint-ST-Wasm.Build.Tests"