Recommending CMake for VC++ users.

IMAO the CMake LLVM build specification has matured enough to supersede
the Visual Studio solution that is distributed with LLVM. It has this
advantages over it:

* Supports all Visual Studio versions which are C++ compliant enough to
  build LLVM (i.e. VS2003 and above). It supports NMake too. No more
  users left in the cold when the maintainers upgrade to a higher
  version of Visual Studio.

* Creates Visual Studio project files which are more complete and
  accurate than those we have now (includes header and tablegen
  definition files, takes library dependence info from a llvm-config
  look-alike implemented as a CMake script).

* (Optional) Supports building all targets, not just X86.

* It is less likely to break, as there are more people updating the
  CMake files, including some LLVM developers.

* It will benefit from further improvements introduced for the
  convenience of the whole LLVM community.

The only downside of this is... well, that CMake is required.

Thus I'm proposing to recommend CMake for Visual C++ users on the LLVM
documentation. This implies removing

which is quite outdated anyways, and putting a pointer on

to the CMake docs with a note about being the recommended build method
for VC++ users.

You can read the LLVM CMake docs on

it is a work on progress, but should work for almost all VC++
users. I'll add details on the next days for covering some corner
cases. Suggestions, grammar corrections, etc welcomed.

If someone thinks that the CMake-generated solution is missing an
important feature the hand-made one has, please describe it and I'll try
to fix the issue.