RFC: MSVC continued

Following up on #6230, #6349, and others. This gets past the stack overflow when compiling `inference.jl` into the system image, but craps out at `pcre.jl` because the preprocessed PCRE headers don't come out correctly from the MSVC preprocessor (it doesn't understand `-dM`). Putting this up so people can look at it, I'll keep thinking about workarounds for the preprocessor issues - can probably copy some of them straight out of the MinGW binaries.

How to run this, assuming you have Visual Studio 2013 (express should work) installed:
- start a command prompt, `cd "C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC"`
- `vcvarsall x86_amd64`
- `cd` to your MSYS installation, whichever folder `sh.exe` is located in (either `bin` or `usr/bin` depending whether you're using MSYS1 or a recent MSYS2 - best to use msysgit here, otherwise you'll likely get an error message due to the MSYS version of `link.exe` showing up first in the path)
- `set MSYSTEM=MINGW32`
- `sh --login`
- `cd` to your Julia directory, checkout this branch
- make sure msys can find 7z on the path
- `ARCH=x86_64 USEMSVC=1 contrib/windows/msys_build.sh`

This downloads the latest Windows nightly binary and uses as many dependencies from it as possible, and downloads an MSVC build of LLVM that I made in early April. I should update that with our latest patches, and build a 32-bit version with VS 2013 at some point.