[LLVM-C] Add LLVM-C.dll to Windows installer package

This is a follow up to D56781, D56774 and D35077 to makes the LLVM-C.dll
file and LLVM-C.lib be installed on Windows, just like LTO.dll and
LTO.lib are.

Patch by Jakob Bornecrantz!

Differential revision: https://reviews.llvm.org/D63717

llvm-svn: 364275