glsl: Move layout(location) checks to AST-to-HIR conversion

glsl: Move layout(location) checks to AST-to-HIR conversion

This will simplify the addition of layout(location) qualifiers for
separate shader objects.  This was validated with new piglit tests
arb_explicit_attrib_location/1.30/compiler/not-enabled-01.vert and
arb_explicit_attrib_location/1.30/compiler/not-enabled-02.vert.

v2: Refactor error checking to check_explicit_attrib_location_allowed
and eliminate the gotos.  Suggested by Paul.

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