Print all BuiltIns inside a block

[spencer-lunarg]

Sometimes BuiltIn are all inside a Block, example

out gl_PerVertex {
    vec4 gl_Position;
    float gl_PointSize;
};

but currently we just print

location : (built-in) ??? (-1)

but now with this PR we print

location : (built-in block) [Position, PointSize]

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It is invalid SPIR-V to mix and match built-in variables and non-built-in variables within a single structure, so we can ensure the entire block has BuiltIns