Update the Visual Studio Integration from user feedback.

This patch removes the MSBuild warnings about options that
clang-cl ignores.  It also adds several additional fields to
the LLVM Configuration options page.  The first is that it
adds support for LLD!  To give the user flexibility though,
we don't want to force LLD to always-on, and if we're not
forcing LLD then we might as well not force clang-cl either.
So we add options that can enable or disable lld, clang-cl,
or any combination of the two.  Whenever one is disabled,
it falls back to the Microsoft equivalent.

Additionally, for each of clang-cl and lld-link, we add a new
configuration setting that allows Additional Options to be
passed for that specific tool only.  This is similar to the
C/C++ > Command Line > Additional Options entry box, but
it serves the use case where a user switches back and forth
between the toolsets in their vcxproj, but where cl.exe
won't accept some options that clang-cl will.  In this case
you can pass those options in the clang-cl additional options
and whenever clang-cl is disabled (or the other toolset is
selected entirely), those options won't get passed at all.

llvm-svn: 340780