Pre-llvm10 does not allow positional argument identifiers

[kquick]

As seen in the test output from GaloisInc/llvm-pretty-bc-parser#233, llvm-pretty will ingest

...
define dso_local i32 @test1(i32) #0 {
...

but will then emit:

...
define default i32 @test1(i32 %0) {
...

where the latter causes a failure when passed to llvm-as version 9:

llvm-as: disasm-test/tests/callbr.pre-llvm10.ll:4:31: error: expected ')' at end of argument list

and it highlights the %0 of the argument list. Prior to llvm-10, it appears that positional argument identifiers are not allowed.