glsl: Only allow `invariant` on shader in/out between stages.

Previously this was special-cased for VS and FS; it never got updated
when geometry shaders came along. Generalize using is_varying_var() so
this won't be broken again with tessellation.

Note that there are two copies of the logic for `invariant`: It can be
present as part of a new declaration, and also as a redeclaration of an
existing variable or block member.

Fixes the four new piglits:
   spec/glsl-1.50/compiler/invariant-qualifier-*.geom

Note for stable: This won't quite pick cleanly due to whitespace and
state->target -> state->stage renames. Should be straightforward
adjustments though.

Cc: "10.0 10.1" <[email protected]>
Signed-off-by: Chris Forbes <[email protected]>
Reviewed-by: Ian Romanick <[email protected]>

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