[Mesa-dev] Split gl_shader in two and clean-ups

[Timothy Arceri]

There are two distinctly different uses of this struct. The first
is to store GL shader objects. The second is to store information
about a shader stage thats been linked.

The only place the new structs overlap is the shader layout fields and
I intend to split that out into a third struct once this series lands.

Having two well defined structs helps code readability and allows the removal
of some unreachable code paths that were the result of confusion between
the two uses.