Add a --enable-clang-plugin-support option to configure.

This will replace the now badly named CLANG_IS_PRODUCTION.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203471 91177308-0d34-0410-b5e6-96231b3b80d8
[bugpoint] Add testcase for r203343.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203472 91177308-0d34-0410-b5e6-96231b3b80d8