linker: Modify cross_validate_outputs_to_inputs to match using explicit locations

linker: Modify cross_validate_outputs_to_inputs to match using explicit locations

This will be used for GL_ARB_separate_shader_objects.  That extension
not only allows separable shaders to rendezvous by location, but it also
allows traditionally linked shaders to rendezvous by location.  The spec
says:

    36. How does the behavior of input/output interface matching differ
        between separable programs and non-separable programs?

        RESOLVED: The rules for matching individual variables or block
        members between stages are identical for separable and
        non-separable programs, with one exception -- matching variables
        of different type with the same location, as discussed in issue
        34, applies only to separable programs.

        However, the ability to enforce matching requirements differs
        between program types.  In non-separable programs, both sides of
        an interface are contained in the same linked program.  In this
        case, if the linker detects a mismatch, it will generate a link
        error.

v2: Make sure consumer_inputs_with_locations is initialized when
consumer is NULL.  Noticed by Chia-I.

v3: Rebase on removal of ir_variable::user_location.

v4: Replace a (stale) FINISHME with some good explanation comments from
Eric.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>