glsl: Generate an interface type for uniform blocks

glsl: Generate an interface type for uniform blocks

If the block has an instance name, add the instance name to the symbol
table instead of the individual fields.

Fixes the piglit test interface-name-access-without-interface-name.vert
for real.

v2: Update the comment before the assertion that interface block
definitions won't generate instructions.  Suggested by Paul Berry.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>