Fix the MSVC build.  MSVC does not support constexpr.
Fix supported architectures on PlatformWindows.

i386, i486, i486sx, and i686 are all indistinguishable as far as
PE/COFF files are concerned.  This patch adds support for all of
these architectures to PlatformWindows.

Differential Revision: http://reviews.llvm.org/D4658
Use llvm Support functions to get the user's home directory.

Assuming that the user's home directory is at ~ is incorrect on
Windows.  This patch delegates the request to LLVM's support
library, which already provides a cross-platform implementation
of this function.

Differential Revision: http://reviews.llvm.org/D4674
Teach LLDB about Windows processes.

This patch creates a simple ProcessWindows process plugin.
The only thing it knows how to do currently is create processes.

Differential Revision: http://reviews.llvm.org/D4681