Fixed isReadonlySymbol check for intersection properties with .valueDeclaration

[Andarist]

fixes #61344

[Andarist]

1. The issue arised because getDeclarationModifierFlagsFromSymbol(symbol) was returning ModifierFlags.Readonly based on the .valueDeclaration.
2. .valueDeclaration is only set on union/intersection properties when it's "uniform"(ish). In this example, one source property symbol had a .valueDeclaration and the other one hadn't (the one from the mapped type hadn't it)
3. but given that, as the existing comment states, the readonly flag is precomputed eagerly for union/intersections - this is really the only thing the compiler has to check in that situation. So I leveraged that for the fix here.

[typescript-bot]

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