[clang-tools-extra] [llvm] [clang-doc] fix bug introduced by asset test (PR #97540)

[Michał Górny via cfe-commits]

mgorny wrote:

This change broke standalone clang builds against installed LLVM, since `LLVM_SHARE_OUTPUT_INTDIR` is not exported by LLVM and therefore resolves to an empty path:

```
ninja: error: mkdir(/clang-doc): Permission denied
ninja: build stopped: .
```

Why are you even declaring the directory inside LLVM when it's only used by Clang?

Please revert.

https://github.com/llvm/llvm-project/pull/97540