Suppress CS3016 on internal COM wrappers without [CLSCompliant(false)] (#1703)

[JeremyKuhne]

Summary

Fixes the dotnet/winforms#14639 fallout from #1703: a consumer that adds a friendly-helper partial to a generated CCW-bearing COM struct (e.g. System.Private.Windows.Core's ITypeInfo.cs) now builds clean.

#1703 marked CCW-bearing COM struct wrappers [CLSCompliant(false)] to silence CS3016 (a Roslyn limitation that flags the array-valued [UnmanagedCallersOnly(CallConvs = new[]{...})] thunk argument even on non-visible types — dotnet/roslyn#68526). But that type-level attribute induces CS3019/CS3021, which Roslyn reports against the type symbol. When a consumer adds their own partial to the struct, the diagnostic lands on the consumer''s file and escapes the generated file''s pragma:

• assembly has no [assembly: CLSCompliant] -> CS3021
• assembly is [assembly: CLSCompliant(true)] -> CS3019

Under TreatWarningsAsErrors this breaks the consumer build.

Fix

Stop emitting [CLSCompliant(false)]. Instead suppress CS3016 directly in the generated file''s header pragma, where the offending thunk lives:

• CS3016 is reported at the thunk method (always in our generated file), so the file-scoped pragma silences it precisely without masking user code.
• Because no attribute is emitted, CS3019/CS3021 can never arise — robust to consumer partials and any assembly CLS posture.
• The suppression is applied to internal projections only (the default). Public projections do not get it: a public COM interop surface is the consumer''s CLS contract to own (and is inherently non-CLS-compliant anyway), so CsWin32 must not unilaterally silence it.

Removes the now-unused CLSCompliantFalse() factory and the CS3019/CS3021 entries from the generated-file pragma.

Tests

• Internal cross-TFM (net472/net10.0) x assembly-CLS-posture matrix asserting no CS3016/CS3019/CS3021.
• Public projection omits the CS3016 suppression (and emits no [CLSCompliant]).
• Friendly-helper-partial regressions with and without an assembly [CLSCompliant] attribute (the winforms#14639 shape).
• A load-bearing test that strips the generated pragma and asserts CS3016 does fire (proving the suppression is real, not vacuous).

Validated end-to-end with a real net472;net10.0 cross-compile consuming the locally-built generator.