For SPIR-V, only export the main function if it is actually declared. Kernel
entry points will often have parameters and more than one kernel declared.
In general, SPIR-V binaries should mostly be compiled as libraries and not as
executables. However, this start code is required so that we can build test
executables.
Note that a call to isSpirV() would emit the code for that function, even though
the call is at comptime. To save that function from being emitted the checks
are just inlined manually.
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