[clang] [PS4, PS5][Driver] Detangle --sysroot and -isysroot (PR #107410)

[Paul T Robinson via cfe-commits]

https://github.com/pogo59 commented:

According to [gcc docs](https://gcc.gnu.org/onlinedocs/gcc/Directory-Options.html) `--sysroot` applies to headers and libraries, and `-isysroot` overrides it for headers only. This is different from the "independence" described for World 2. That is, if we have only `--sysroot` it should apply to both.

https://github.com/llvm/llvm-project/pull/107410