MSL: Add reversed depth viewport emulation

[vkedwardli]

This add an opt-in MSL backend option to emulate reversed depth viewports by inverting clip-space Z in vertex-like shaders.

The option originated from the MoltenVK workaround discussion in KhronosGroup/MoltenVK#2760
On some affected Metal implementations, passing a reversed depth viewport directly to Metal can fail. The intended consumer is MoltenVK, which can normalize the Metal viewport depth range and ask SPIRV-Cross to emulate the missing reversed-depth transform in generated MSL

When enabled, SPIRV-Cross adds a constant uint& mask argument and emits:

position.z = position.w - position.z

for viewports whose mask bit is set.

Changes:

• CompilerMSL::Options::emulate_reversed_depth_viewport
• CompilerMSL::Options::reversed_depth_viewport_buffer_index
• C API options for both fields
• MSL emission for the clip-space Z inversion in vertex-like shaders

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