glsl: Propagate explicit binding information from AST to IR.

glsl: Propagate explicit binding information from AST to IR.

Rather than creating a new "binding" field in ir_variable, we reuse
constant_value since the linker code for handling uniform initializers
uses that.

Since UBOs and samplers can't otherwise have initializers/constant
values, there shouldn't be a conflict.

v2: Propagate the new binding variable around too.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>