Make DebugLinePriority test valid SPIR-V (#3672)
A recent version of SPIRV-Tools found several issues with the test, such
as `DebugTypeFunction` having the wrong return type operand and
`DebugTypeBasic` missing the flags operand.
Original commit:
https://github.com/KhronosGroup/SPIRV-LLVM-Translator/commit/bf469923a25d484
