Fix XamlDirective stack overflow exception

[ThomasGoulet73]

Fixes #10313

Description

I changed MemberReflector.UnknownReflector to be thread safe by initializing the property from the static constructor instead of doing the lazy initialization manually. The static constructor of a type is guaranteed to be called only once.

See the issue for an explanation of the bug and how it's hit.

Customer Impact

Could fix a crash or weird behavior at runtime but it's very hard to hit (See issue).

Regression

No.

Testing

Used the repro in the issue which crashed before this PR and passes with this PR.

Risk

Low.

Microsoft Reviewers: Open in CodeFlow

[miloush]

Wouldn't it be enough if you did

private static MemberReflector s_UnknownReflector;
internal static MemberReflector UnknownReflector => s_UnknownReflector ??= CreateUnknownReflector();

[h3xds1nz]

Is there a reason why we're not keeping lazy-init, just making it TS instead?

[ThomasGoulet73]

It was lazy because static fields only get assigned when necessary and in this case CreateUnknownReflector was only called on the first call to UnknownReflector and it was thread-safe because static constructors are thread safe.

I pushed a commit to change it to make the lazy init explicit and more clear.