Add top-level Clang flag -f(no-)sanitize-address-zero-base-shadow that makes AddressSanitizer use bottom of the address space for the shadow memory. On Linux it can be used with -fPIE/-pie to improve performance.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@172974 91177308-0d34-0410-b5e6-96231b3b80d8