(I'm using chandlerc and rnk mostly for the CMake stuff, but naturally they are unfamiliar with the dotest stuff, so that's fair game if anyone else has any comments or concerns)
Previously, CMake was invoking the test runner and not specifying what architecture to use when building test executables. The Makefiles for the test executables then had logic to choose x64 by default. This doesn't work on Windows because the test compiler would then try to link against the 64-bit MSVCRT and not find them since only the 32-bit MSVCRT was in the path.
This patch addresses this by figuring out, at CMake time, whether or not you are building LLDB with a 64 or 32-bit toolchain. Then, it explicitly passes this value to the test runner, causing the test runner to build tests whose architecture matches that of LLDB itself. This can still be overridden by setting the CMake variable LLDB_TEST_EXECUTABLE_ARCH=(x64|x86)