[Yexuan Xiao (YexuanXiao)]

Fixes #1647.

winrt_numerics.ixx includes windowsnumerics.impl.h in the module purview as its implementation, but windowsnumerics.impl.h transitively includes other standard library headers, causing some standard library definitions to also become part of the winrt_numerics implementation. The global module fragment includes directxmath.h, directxmath.h also transitively includes standard library headers, and unlike windowsnumerics.impl.h. This causes winrt_numerics.ixx to be expanded into the following pseudocode:

module;
#include <stdheader1>
export module winrt_numerics;
#include <stdheader2>

Once stdheader1 declares a class A but A is defined in stdheader2, this violates the standard's requirements, causing the build to fail. Since MSVC is known to be non-conforming on this issue and uses a relaxed strategy, this problem was not discovered in the previous implementation.

Link to windowsnumerics.impl.h of mingw-w64.