[llvm] Treat `''` and `""` in Gnu-tokenized response files as an empty string argument. (PR #187566)

[Tony Allevato via llvm-commits]

allevato wrote:

@jyknight Added those tests and a couple others (e.g., unterminated quote at EOF and newlines). Manually verified that the behavior matches a `gcc` invocation. PTAL.

https://github.com/llvm/llvm-project/pull/187566