Preserve OpenCL as_<type>() calls during re-writing

[ChrisCummins]

livclc defines the family of as_char(), as_int(), etc calls as macros: https://github.com/llvm-mirror/libclc/blob/master/generic/include/clc/as_type.h

This means that the clgen re-writer will inline them to __builtin_astype() calls. We shouldn't do this.

The fix may be as simple as #undef-ing all of those libclc macros.