[CMake] Add test-depends target to build dependencies of check-all

This is just another convenience target for bots to use. It enables isolation of building and testing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@262494 91177308-0d34-0410-b5e6-96231b3b80d8

CMakeLists.txt

@@ -727,6 +727,7 @@ if( LLVM_INCLUDE_TESTS )
     DEPENDS ${LLVM_LIT_DEPENDS}
     ARGS ${LLVM_LIT_EXTRA_ARGS}
     )
+  add_custom_target(test-depends DEPENDS ${LLVM_LIT_DEPENDS})
 endif()
 
 if (LLVM_INCLUDE_DOCS)