Vulkan: Have desktop shaders respect precision qualifiers.

Sets highp defaults for the appropriate types, for all stages,
and turns on precision qualifiers for non-ES shaders.  Required
fixing some qualifier orders for desktop built-in declarations
for pre-420 shaders.

Test/baseResults/spv.structAssignment.frag.out

@@ -34,6 +34,8 @@ Linked fragment stage:
                               Name 44  "coord"
                               Name 49  "foo"
                               Decorate 40(samp2D) DescriptorSet 0
+                              Decorate 44(coord) RelaxedPrecision
+                              Decorate 45 RelaxedPrecision
                2:             TypeVoid
                3:             TypeFunction 2
                6:             TypeInt 32 1