Front-end: Warn for likely missed change in default precisions.

This is part of the change to have desktop shaders respect precision
qualifiers on Vulkan, but since the defaults are all highp, and that's
different from ES fragment shaders, detect likely cases and warn about
them (but being careful to not be too noisy if it's unlikely to be a
problem).

Test/baseResults/spv.aggOps.frag.out

@@ -1,5 +1,7 @@
 spv.aggOps.frag
 Warning, version 450 is not yet complete; most version-specific features are present, but some are missing.
+WARNING: 0:4: '' : all default precisions are highp; use precision statements to quiet warning, e.g.:
+         "precision mediump int; precision highp float;" 
 
 
 Linked fragment stage:

Test/baseResults/spv.structAssignment.frag.out

@@ -1,4 +1,7 @@
 spv.structAssignment.frag
+WARNING: 0:6: '' : all default precisions are highp; use precision statements to quiet warning, e.g.:
+         "precision mediump int; precision highp float;" 
+
 
 Linked fragment stage:
 
@@ -33,6 +36,10 @@ Linked fragment stage:
                               Name 40  "samp2D"
                               Name 44  "coord"
                               Name 49  "foo"
+                              MemberDecorate 8(lunarStruct1) 0 RelaxedPrecision
+                              MemberDecorate 9(lunarStruct2) 0 RelaxedPrecision
+                              MemberDecorate 10(lunarStruct3) 1 RelaxedPrecision
+                              Decorate 16 RelaxedPrecision
                               Decorate 40(samp2D) DescriptorSet 0
                               Decorate 44(coord) RelaxedPrecision
                               Decorate 45 RelaxedPrecision

Test/baseResults/vulkan.frag.out

@@ -33,6 +33,8 @@ ERROR: 0:67: 'uniform' : no qualifiers allowed for function return
 ERROR: 0:69: 'non-opaque uniforms outside a block' : not allowed when using GLSL for Vulkan 
 ERROR: 0:73: 'texture' : no matching overloaded function found 
 ERROR: 0:74: 'imageStore' : no matching overloaded function found 
+WARNING: 0:82: '' : all default precisions are highp; use precision statements to quiet warning, e.g.:
+         "precision mediump int; precision highp float;" 
 ERROR: 0:91: 'call argument' : sampler constructor must appear at point of use 
 ERROR: 0:92: 'call argument' : sampler constructor must appear at point of use 
 ERROR: 0:93: ',' : sampler constructor must appear at point of use 

Test/spv.merge-unreachable.frag

@@ -1,4 +1,5 @@
 #version 450
+precision mediump int; precision highp float;
 layout(location=1) in highp vec4 v;
 void main (void)
 {

Test/spv.structAssignment.frag

@@ -1,5 +1,7 @@
 #version 140
 
+precision mediump int;
+
 uniform sampler2D samp2D;
 in mediump vec2 coord;