glsl: Basic support for built-in intrinsics.

glsl: Basic support for built-in intrinsics.

Fix the linker to deal with intrinsic functions which are undefined
all the way down to the driver back-end, and introduce intrinsic
definition helpers in the built-in generator.

We still need to figure out what kind of interface we want for drivers
to communicate to the GLSL front-end which of the supported intrinsics
should use a default GLSL implementation and which should use a
hardware-specific override.  As there's no default GLSL implementation
for atomic ops, this seems like something we can worry about later on.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
v2: Define local helper function to generate ir_call nodes in the
    builtin generator.