MSL: mark BuiltInFragCoord as implicitly used for subpass reads
MSL: mark BuiltInFragCoord as implicitly used for subpass reads

Without this patch, `gl_FragCoord` is not output for subpass reads in certain cases where `gl_FragCoord` is not used elsewhere in the shader.

I'm not sure why this isn't caught by existing tests (e.g. [input-attachment.frag](shaders-msl/frag/input-attachment.frag)), but I encountered this issue in code generated by DXC and passed through spire-opt.
KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross