[lldb][CMake] Infer `Clang_DIR` if not passed explicitly

Summary:
If we only get `LLVM_DIR` and find Clang in the same provided build-tree, automatically infer `Clang_DIR` like this:

```
LLVM_DIR = /path/to/build-llvm/lib/cmake/llvm
Clang_DIR = /paht/to/build-llvm/lib/cmake/clang
```

Reviewers: JDevlieghere, jingham, xiaobai, compnerd, labath

Reviewed By: JDevlieghere, labath

Subscribers: mgorny, lldb-commits, #lldb

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D65798

llvm-svn: 372210