Update instructions after change in clang buildsystem

[aaron.ballman]

Rather than make this a dependent revision, this sort of thing really should be rolled into the original patch so we don't lose track of parts of a revision during the review process. Dependent revisions are for more substantive changes that are orthogonal (but related) to one another.

[steveire]

I use separate git repos. This patch is in LLVM.git, and the other is in clang.git.

docs/GettingStarted.rst

#. Checkout Clang:
    * ``cd where-you-want-llvm-to-live``
    * ``cd llvm/tools``
    * ``svn co http://llvm.org/svn/llvm-project/cfe/trunk clang``
 
 #. Checkout Extra Clang Tools **[Optional]**:
 
    * ``cd where-you-want-llvm-to-live``
    * ``cd llvm/tools/clang/tools``
-   * ``svn co http://llvm.org/svn/llvm-project/clang-tools-extra/trunk extra``
+   * ``svn co http://llvm.org/svn/llvm-project/clang-tools-extra/trunk clang-tools-extra``
 
 #. Checkout LLD linker **[Optional]**:
 
    * ``cd where-you-want-llvm-to-live``
    * ``cd llvm/tools``
    * ``svn co http://llvm.org/svn/llvm-project/lld/trunk lld``
 
 #. Checkout Polly Loop Optimizer **[Optional]**: