CMake: set stack size to 2MB for MSVC builds

Compiling under Visual C++ 2012 with the default stack size of 1MB, the stack
overflows at a depth of 216 template instantiations, well before the 256
default limit. This patch modifies the default MSVC stack size to 2MB.

Patch by Yaron Keren!

llvm-svn: 192904