CMake: set LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN at configure time

Looks like this is needed to build recent LLVM on Ubuntu 14.04.

Author: fredriss

utils/swift_build_support/swift_build_support/cmake.py

@@ -132,6 +132,8 @@ def common_options(self):
         define("CMAKE_CXX_COMPILER:PATH", toolchain.cxx)
         define("CMAKE_LIBTOOL:PATH", toolchain.libtool)
 
+        define("LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN", "ON")
+
         if args.cmake_generator == 'Xcode':
             define("CMAKE_CONFIGURATION_TYPES",
                    "Debug;Release;MinSizeRel;RelWithDebInfo")