SPIR-V doesn't support variadic functions, though we make an exception
for `printf`.
If we don't error, we generate invalid SPIR-V because the backend has no
idea how to codegen vararg functions as it is not described in the spec.
We get asm like this:
```
%27 = OpFunction %6 None %7
%28 = OpFunctionParameter %4
; -- End function
```
The above asm is totally invalid, there's no `OpFunctionEnd` and it
causes crashes in downstream tools like `spirv-as` and `spirv-link`.
We already have many `printf` tests locking down that this doesn't break
`printf`, it was already handled elsewhere at the time the error check
runs.
Note the SPIR-V Translator does the same thing, see
[here](https://github.com/KhronosGroup/SPIRV-LLVM-Translator/pull/2703).
---------
Signed-off-by: Nick Sarnie <nick.sarnie@intel.com>
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