clang: Do not try to build clang-tblgen in LLVM's build path.

pkgs/development/compilers/llvm/clang-tablegen-dir.patch

@@ -0,0 +1,9 @@
+--- a/utils/TableGen/CMakeLists.txt       (revision 190146)
++++ b/utils/TableGen/CMakeLists.txt       (working copy)
+@@ -1,4 +1,5 @@
+ set(LLVM_LINK_COMPONENTS Support)
++set(LLVM_TOOLS_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR})
+ 
+ add_tablegen(clang-tblgen CLANG
+   ClangASTNodesEmitter.cpp
+

pkgs/development/compilers/llvm/clang.nix

@@ -10,7 +10,8 @@ stdenv.mkDerivation {
 
   buildInputs = [ perl llvm groff cmake libxml2 python ];
 
-  patches = stdenv.lib.optional (stdenv.gcc.libc != null) ./clang-purity.patch;
+  patches = [ ./clang-tablegen-dir.patch ] ++
+            stdenv.lib.optional (stdenv.gcc.libc != null) ./clang-purity.patch;
 
   cmakeFlags = [
     "-DCLANG_PATH_TO_LLVM_BUILD=${llvm}"