Based on OpenCL.DebugInfo.100 spec, if the function has no return
value, 'Return Type' operand of DebugTypeFunction is OpTypeVoid.

Bug: b/148401179
Change-Id: Ib29c7a569ff2ec24ba4667523766b0bdb0d6f260
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/48128
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Tested-by: Jaebaek Seo <jaebaek@google.com>