[clang] [clang][lit] Fix spirv-tools-err.c when LIT_USE_INTERNAL_SHELL=0 (PR #185876)

[Aiden Grossman via cfe-commits]

https://github.com/boomanaiden154 commented:

Why are you still using the external shell?

The plan is to deprecate it in LLVM 23 and remove it by LLVM 24.

https://github.com/llvm/llvm-project/pull/185876