[clangd] Skip extra round-trip in parsing args in debug builds. NFC

This is a clever cross-cutting sanity test for clang's arg parsing I suppose.
But clangd creates thousands of invocations, ~all with identical trivial
arguments, and problems with these would be caught by clang's tests.
This overhead accounts for 10% of total unittest time!

Differential Revision: https://reviews.llvm.org/D125169