Updated getting started guide for visual studio + cmake

[aaron.ballman]

Summary

It took me a while to track this down and I figured I'd save someone else the time. By default, CMake uses the 32-bit toolchain on Windows, even if generating a 64-bit solution. Given the size of Clang's code base, this can lead to quite a few link errors with the linker running out of memory. If you pass -Thost=x64 when generating the Visual Studio solution, then the resulting project files will use the 64-bit toolchain and the toolchain no longer runs out of memory.

The same wording is being used in D33546 for LLVM's side of things.

[aaron.ballman]

Commit in r303913

www/get_started.html

     <li><tt>mkdir build</tt> (for building without polluting the source dir)</li>
     <li><tt>cd build</tt></li>
     <li>If you are using Visual Studio 2013:  <tt>cmake -G "Visual Studio 12" ..\llvm</tt></li>
+    <li>By default, the Visual Studio project files generated by CMake use the
+     32-bit toolset. If you are developing on a 64-bit version of Windows and
+     want to use the 64-bit toolset, pass the ``-Thost=x64`` flag when
+     generating the Visual Studio solution. This requires CMake 3.8.0 or later.</li>
     <li>See the <a href="http://www.llvm.org/docs/CMake.html">LLVM CMake guide</a> for
         more information on other configuration options for CMake.</li>
     <li>The above, if successful, will have created an LLVM.sln file in the