[ClangImporter] Add the sysroot directly to the clang Driver used to find the libc header path
[ClangImporter] Add the sysroot directly to the clang Driver used to find the libc header path

`getToolChain()` and `AddClangSystemIncludeArgs()` don't parse the arguments to set the
sysroot, so set it directly instead. Also, remove a redundant check for a valid directory.

Fixes #60224

apple/swift,apple/swift,glessard/swift,roambotics/swift,apple/swift,apple/swift,roambotics/swift,roambotics/swift,roambotics/swift,benlangmuir/swift,benlangmuir/swift,benlangmuir/swift,apple/swift,benlangmuir/swift,glessard/swift,benlangmuir/swift,glessard/swift,glessard/swift,apple/swift,roambotics/swift,roambotics/swift,benlangmuir/swift,glessard/swift,glessard/swift