glsl: use an enum for AMD_conservative_depth layout qualifiers

glsl: use an enum for AMD_conservative_depth layout qualifiers

The main idea behind this is to free some bits in the flags.q
struct because currently all 64-bits are used and we can't
add more layout qualifiers without reaching a static assert.

In order to do that (mainly for ARB_bindless_texture), use an
enumeration for the AMD_conservative_depth layout qualifiers
because it's forbidden to declare more than one depth qualifier
for gl_FragDepth.

Note that ast_type_qualifier::merge_qualifier() will prevent
using duplicate layout qualifiers by returning a compile-time
error.

No piglit regressions found (including compiler tests) with
RX480 on RadeonSI.

v2: use a switch case

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Andres Gomez <agomez@igalia.com> (v1)