identifier "size_t" is undefined, with clang 18

[Colengms]

With clang 18(.1.3), I'm seeing an issue with size_t, which doesn't repro with earlier versions of clang. (15-17).

The issue likely has something to do with how the clang-18 headers decide whether or not to define size_t. Some IntelliSense predefines may be confusing it, making it think size_t is already defined.