CodeGen: handle llvm.used properly for COFF

`llvm.used` contains a list of pointers to named values which the
compiler, assembler, and linker are required to treat as if there is a
reference that they cannot see.  Ensure that the symbols are preserved
by adding an explicit `-include` reference to the linker command.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@323017 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed