Calling a Kernel Function in a Separately Compiled Program

Creating this issue from a discussion in a SPIRV-LLVM-Translator PR review:

https://github.com/KhronosGroup/SPIRV-LLVM-Translator/pull/1149#discussion_r707417308

In short, the question is: can a function in one program call a kernel function in another program?

Here is the example from the PR discussion:

// source 1
void foo(global int *_B);     
void kernel A(global int *B) {
  foo(B);                     
}
// source 2
void kernel foo(global int *B) {
  B[0] = 1;
}

This came about because the SPIR-V spec states that "It is invalid for any function to be targeted by both an OpEntryPoint instruction and an OpFunctionCall instruction." The SPIR-V LLVM Translator PR solves this by introducing an "entry point wrapper" around a kernel function that is also called as an ordinary function then fixing up the calls to the kernel function in the program. This works for callers in the same program, but it won't work for callers in other programs.

Start with the linked SPIRV-LLVM-Translator PR discussion and the quoted SPIR-V rule about OpEntryPoint and OpFunctionCall. Determine whether a kernel function in one separately compiled program may be called from another, and document a definitive answer or the required specification clarification.