Add a new build step: build asan RTL with clang

git-svn-id: http://address-sanitizer.googlecode.com/svn/trunk@1857 3c3640eb-14a6-2bb8-0b94-e9e73afe33f5
Fix the clang-cl path to work from .bat

git-svn-id: http://address-sanitizer.googlecode.com/svn/trunk@1858 3c3640eb-14a6-2bb8-0b94-e9e73afe33f5