[ms] Add new option to llvm-lib: /llvmlibempty

Summary:
Add a new option (/llvmlibempty). If passed and llvm-lib does not give an error, it will create a valid output archive even if empty.

By default, llvm-lib mimicks lib.exe: if given no input files, it doesn't create its output file at all. This is incompatible with some build systems, so we add a command-line option to toggle this compatibility behavior.

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D78894