[llvm] Avoid exposing password and token from git repositories (PR #105220)

[Danila Malyutin via llvm-commits]

danilaml wrote:

Adding `-DLLVM_FORCE_VC_REPOSITORY` doesn't achieve the intended effect. It overrides the repository but also no revision is generated either, i.e. VCSRevision.h becomes
```c
#undef LLVM_REVISION
#define LLVM_REPOSITORY "test"
```
Considering `LLVM_REPOSITORY` is only really used by lld, clang and flang, it seems to create unnecessary friction with no easy workaround for other projects.

https://github.com/llvm/llvm-project/pull/105220