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.)