[Driver] Improve the `isPathUnderSysroot()` function. Now it returns a
correct result even if checking paths: a) symlinks and/or b) contains relative
parts like /dir1/dir2/../dir2.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@211772 91177308-0d34-0410-b5e6-96231b3b80d8