Add empty shell of llvm-cvtres.

This marks the beginning of an effort to port remaining
MSVC toolchain miscellaneous utilities to all platforms.

Currently clang-cl shells out to certain additional tools
such as the IDL compiler, resource compiler, and a few
other tools, but as these tools are Windows-only it
limits the ability of clang to target Windows on other
platforms.  having a full suite of these tools directly
in LLVM should eliminate this constraint.

The current implementation provides no actual functionality,
it is just an empty skeleton executable for the purposes
of making incremental changes.

Differential Revision: https://reviews.llvm.org/D32095
Patch by Eric Beckmann (ecbeckmann@google.com)

llvm-svn: 301004