[cmake] Remove use of deprecated generator expression. NFC

Use $<CONFIG> instead of $<CONFIGURATION>, since the latter has been
deprecated since CMake 3.0, and the former is entirely equivalent.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@357338 91177308-0d34-0410-b5e6-96231b3b80d8

cmake/modules/LLVMExternalProjectUtils.cmake

@@ -5,7 +5,7 @@ include(ExternalProject)
 function(llvm_ExternalProject_BuildCmd out_var target bin_dir)
   cmake_parse_arguments(ARG "" "CONFIGURATION" "" ${ARGN})
   if(NOT ARG_CONFIGURATION)
-    set(ARG_CONFIGURATION "$<CONFIGURATION>")
+    set(ARG_CONFIGURATION "$<CONFIG>")
   endif()
   if (CMAKE_GENERATOR MATCHES "Make")
     # Use special command for Makefiles to support parallelism.