glsl: Translate the AST for uniform blocks into some IR structures.

glsl: Translate the AST for uniform blocks into some IR structures.

We're going to need this structure to cross-validate the uniform
blocks between shader stages, since unused ir_variables might get
dropped.  It's also the place we store the RowMajor qualifier, which
is not part of the GLSL type (since that would cause a bunch of type
equality checks to fail).

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>