[llvm] cmake: do not force building with the install name dir (PR #65656)

[Raul Tambre via llvm-commits]

tambry wrote:

Could you update the commit message to reflect that it's no longer undocumented?
```
cmake: allow disabling building with the install name directory

When custom install names and rpaths setups are used they may not
work in the build tree as-is (namely when using absolute paths for
install names in order to avoid rpath juggling in downstream projects).

Add a flag for opting out of this behaviour.

See: https://reviews.llvm.org/D42463
```

https://github.com/llvm/llvm-project/pull/65656