CompilerMSL add emit_custom_functions() function.
CompilerMSL restrict use of as_type<> cast to necessary conditions.
CompilerMSL refactor get_declared_struct_member_size() and
get_declared_struct_member_alignment() functions, and remove
unnecessary get_declared_type_size() functions.
Add test shaders-msl/vulkan/frag/spec-constant.vk.frag.
