[clang][docs] Improve "Obtaining Clang" section

The documentation is written relatively to
`clang-llvm`, not the root repository directory.
Therefore, the `llvm-project` repo needs to be
cloned into the existing directory `clang-llvm`.
As cloning into an existing directory is only
allowed if the directory is empty, I added
`mkdir ~/clang-llvm` to make the intent of
creating an empty directory explicit.

Author: robincaloudis

clang/docs/LibASTMatchersTutorial.rst

@@ -22,8 +22,8 @@ started guide <https://llvm.org/docs/GettingStarted.html>`_.
 
 .. code-block:: console
 
-      cd ~/clang-llvm
-      git clone https://github.com/llvm/llvm-project.git
+      mkdir ~/clang-llvm && cd ~/clang-llvm
+      git clone https://github.com/llvm/llvm-project.git .
 
 Next you need to obtain the CMake build system and Ninja build tool.