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

With the fixes to the building of LLVM-C.dll in D56781 this should now
be safe to land. 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: 352250