Fix OpVariable ordering violation in NonSemantic debug info (#3692)
SPIR-V spec section 2.16.1 requires OpVariable instructions to be first
in the entry block of a function.
The translator was inserting `DebugFunctionDefinition` before the fix.
Original commit:
https://github.com/KhronosGroup/SPIRV-LLVM-Translator/commit/39e94f8985301fb