[clang] [clang][lex] Fix non-portability diagnostics with absolute path (PR #74782)

[Jeremy Wong via cfe-commits]

jrmwng wrote:

@jansvoboda11 One thing is special.

The path ``C:\GitHub`` is a symbolic link to another harddisk.

```
<SYMLINKD>     GitHub [F:\GitHub]
```

https://github.com/llvm/llvm-project/pull/74782