Make check-clang depend on the LLVM split-file utility

Fixes a recently added test that has this dependency. IMO this utility
is generally useful, so we should go ahead and take the new dependency.

clang/test/CMakeLists.txt

@@ -124,6 +124,7 @@ if( NOT CLANG_BUILT_STANDALONE )
     llvm-readobj
     llvm-symbolizer
     opt
+    split-file
     yaml2obj
     )
 

llvm/utils/gn/secondary/clang/test/BUILD.gn

@@ -164,6 +164,7 @@ group("test") {
     "//llvm/tools/llvm-readobj:symlinks",
     "//llvm/tools/llvm-symbolizer:symlinks",
     "//llvm/tools/opt",
+    "//llvm/tools/split-file",
     "//llvm/utils/FileCheck",
     "//llvm/utils/count",
     "//llvm/utils/llvm-lit",