Map nullptr type to void* instead of the invalid type name null

[tannergooding]

Fixes #601.

A macro such as #define VK_NULL_HANDLE nullptr was resolving its CXType_NullPtr type to the literal string null, so --generate-macro-bindings emitted uncompilable code:

[NativeTypeName("#define VK_NULL_HANDLE nullptr")]
public static readonly null VK_NULL_HANDLE = null;

null is not a valid type name. This maps CXType_NullPtr to void* instead, which is exactly how an equivalent #define X ((void*)0) macro is already handled -- it also correctly triggers the unsafe modifier on the containing class via the existing IsUnsafe check:

public static unsafe partial class Methods
{
    [NativeTypeName("#define VK_NULL_HANDLE nullptr")]
    public static readonly void* VK_NULL_HANDLE = null;
}

Added NullPtrMacroBindingTest covering the macro-binding case. Full generator suite is green (3719 passed, 0 failed) with zero golden churn.

Note

This PR was authored with the assistance of Copilot.