Front-end: Implement 2nd task of issue #400; precision of result and operation.

From the ES spec + Bugzilla 15931 and GL_KHR_vulkan_glsl:
- Update precision qualifiers for all built-in function prototypes.
- Implement the new algorithm used to distinguish built-in function
  operation precisions from result precisions.
Also add tracking of separate result and operation precisions, and
use that in generating SPIR-V.
(SPIR-V cares about precision of operation, while the front-end
cares about precision of result, for propagation.)

Test/baseResults/pointCoord.frag.out

@@ -14,7 +14,7 @@ Shader version: 100
 0:9        true case
 0:10        move second child to first child (temp highp 4-component vector of float)
 0:10          'color' (temp highp 4-component vector of float)
-0:10          texture (global lowp 4-component vector of float)
+0:10          texture (global lowp 4-component vector of float, operation at mediump)
 0:10            'sampler' (uniform lowp sampler2D)
 0:10            'gl_PointCoord' (gl_PointCoord mediump 2-component vector of float PointCoord)
 0:9        false case
@@ -50,7 +50,7 @@ Shader version: 100
 0:9        true case
 0:10        move second child to first child (temp highp 4-component vector of float)
 0:10          'color' (temp highp 4-component vector of float)
-0:10          texture (global lowp 4-component vector of float)
+0:10          texture (global lowp 4-component vector of float, operation at mediump)
 0:10            'sampler' (uniform lowp sampler2D)
 0:10            'gl_PointCoord' (gl_PointCoord mediump 2-component vector of float PointCoord)
 0:9        false case