Enable passing external LDFLAGS to the NSS build system for Windows ASAN builds
explicit
Build on clang-cl with MSVC 2013 with fallback emulation

We officially test MSVC2013 builds now, so it makes sense to
emulate the same compiler when building with clang-cl.  Also,
we need to build with fallback mode, since clang-cl doesn't
still support SEH.
Package the clang-cl runtime library for ASAN on Windows ASAN builds