Build LLVM-C.dll by default on windows and enable in release package

As it says in the subject, should have gone long enough now that this
should be safe. This will greatly simplify dealing with LLVM for people
that just want to use the C API on windows. This is a follow up from
D35077.

Patch by Jakob Bornecrantz!

Differential revision: https://reviews.llvm.org/D56774

llvm-svn: 351324