Add an option to disable plugins in clang.

An option with the same name already exists in the makefile build.

The name CLANG_IS_PRODUCTION is historical. We should probably change it, but
should change the configure build at the same time.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@203325 91177308-0d34-0410-b5e6-96231b3b80d8