[PATCH] [ASan tests] Use clang-cl to build tests on Windows

[Reid Kleckner]

Two things:

1. Should we add -o to clang-cl?  MSVC ignores it silently, and CMake
always uses it when it does its initial test compile.
2. Should we even use clang-cl when building compiler-rt?  Do we have lots
of CFLAGS in compiler-rt?  We will either have to expose them as core
clang-cl options, or switch to the regular clang driver.  Do you need this
to handle /MT or /MD?

http://reviews.llvm.org/D3680