Update CMakeList template for benchmarks

Author: natecook1000

benchmark/scripts/generate_harness/CMakeLists.txt_template

@@ -67,9 +67,18 @@ if(NOT SWIFT_LIBRARY_PATH)
   set(SWIFT_LIBRARY_PATH "${tmp_dir}/lib/swift")
 endif()
 
-runcmd(COMMAND "xcrun" "-toolchain" "${SWIFT_DARWIN_XCRUN_TOOLCHAIN}" "-f" "clang"
-       VARIABLE CLANG_EXEC
-       ERROR "Unable to find Clang driver")
+# If the CMAKE_C_COMPILER is already clang, don't find it again,
+# thus allowing the --host-cc build-script argument to work here.
+get_filename_component(c_compiler ${CMAKE_C_COMPILER} NAME)
+
+if(${c_compiler} STREQUAL "clang")
+  set(CLANG_EXEC ${CMAKE_C_COMPILER})
+else()
+  runcmd(COMMAND "xcrun" "-toolchain" "${SWIFT_DARWIN_XCRUN_TOOLCHAIN}" "-f" "clang"
+         VARIABLE CLANG_EXEC
+         ERROR "Unable to find Clang driver")
+endif()
+
 
 # You have to delete CMakeCache.txt in the swift build to force a
 # reconfiguration.