[clang] [clang-tools-extra] Fix issue clangd issue 503 (clangd/clangd/issues/503) (PR #192882)

[João Távora via cfe-commits]

joaotavora wrote:

oops,  I added the flag to make it configurable (i.e. non-realpath-resolving, but I forgot to actually look it up.

https://github.com/llvm/llvm-project/pull/192882