Improper read_image built-ins attribute.

A bunch of read_imageXX bult-ins declared in opencl_1.2.h and opencl.h has __attribute__((const)) (translated to readnone in LLVM IR) which instructs the compiler what these functions do not access memory which is wrong by definition. Proper attribute is __attribute__((pure)) that means the functions do not modify memory. It seem both (SPIR 1.2 and SPIR-V) generators have this bug.

Start with the read_imageXX declarations in opencl_1.2.h and opencl.h, then inspect the SPIR 1.2 and SPIR-V generators that produce them. Verify how __attribute__((const)) is translated to LLVM IR and update the declarations so the generated functions use the intended memory-effect attribute. Done means both generators produce correct attributes for these built-ins.

Written by the indexing model from the issue text.