mesa: Move shader compiler API code to shaderapi.c

mesa: Move shader compiler API code to shaderapi.c

There was nothing ir_to_mesa-specific about this code, but it's not
exactly part of the compiler's core turning-source-into-IR job either.

v2: Split from the ir_to_mesa to glsl/ commit, avoid renaming the sh
    variable.

Acked-by: Paul Berry <stereotype441@gmail.com> (v1)
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>