Fix ValueLazy caching being defeated by readonly fields

[tannergooding]

ValueLazy<T> is a mutable struct: its Value getter computes the value on first access and clears the factory so subsequent accesses return the cached value. Every field storing one was declared readonly, so each access operated on a defensive copy and the cached result was discarded -- the factory ran again on every access, defeating the memoization the type exists to provide. This is the classic mutable-struct-in-a-readonly-field bug.

Dropping readonly from all 326 ValueLazy<T> fields (170 files) lets the cached value persist. The factories are idempotent, so behavior is unchanged; access to lazy members (spellings, parents, translation units, etc.) simply no longer recomputes each time -- which avoids repeated native clang_* calls, GetOrCreate lookups, and string marshaling.

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Measured on a representative real-world workload -- the d3d12 generation from terrafx/terrafx.interop.windows against the Windows SDK:

• Wall-clock: 4404 -> 4230 ms median (~4-5%), with clean separation (baseline min 4342 > fixed max 4241).
• Allocations: 284.9 -> 280.2 MB (~1.6%) under full optimization.

I audited the rest of the runtime, generator, and interop layers for the same pattern: the descriptor structs (StructDesc, ValueDesc, FunctionOrDelegateDesc, etc.) are mutable but only ever used as locals or by-value parameters, and LazyList is a sealed class -- ValueLazy was the only type caught in this trap.

All 3706 tests pass with a 0-warning build.