Add an option to use the MSVC linker to link LTO-generated object files.

This patch defines a new command line option, /MSVCLTO, to LLD.
If that option is given, LLD invokes link.exe to link LTO-generated
object files. This is hacky but useful because link.exe can create
PDB files.

Differential Revision: https://reviews.llvm.org/D29526

llvm-svn: 294234