glsl: Store the precision for a function return type

glsl: Store the precision for a function return type

The precision for a function return type is now stored in
ir_function_signature. This will later be useful to implement mediump
to float16 lowering. In the meantime it is also useful to catch errors
where a function is redeclared with a different precision.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>