[llvm] [Support] Resolve symlinks in `getMainExecutable()` on Windows (PR #76304)

[Mike Hommey via llvm-commits]

glandium wrote:

This has the unfortunate consequence that if you have llvm-config in a path with spaces and call it with the short path, the paths it will emit will contain spaces, when they didn't before.

That is, before:
```
$ mkdir -p "FOO BAR/bin"
$ cp llvm-config.exe "FOO BAR/bin"
$ ./FOOBAR~1/llvm-config.exe --bindir
(...)\FOOBAR~1\bin
```
But now:
```
$ ./FOOBAR~1/llvm-config.exe --bindir
(...)\FOO BAR\bin
```

It's not too much of a problem with `--bindir`, but with e.g. `--libs`, that means we now get a spaces-separated list of files separated by spaces, which doesn't quite work (which using the shortname before worked around).

https://github.com/llvm/llvm-project/pull/76304