Avoid calling strtoll and atoll in asan_str_test.cc on Windows for now

Summary:
These two functions are unavailable on MSVC2012, which breaks building the
ASAN tests with MSVC2012.  Since the tests required to run these functions
are disabled on Windows for now, avoid building them to fix the MSVC2012
builds.

Test Plan: This is needed in order to fix building the ASAN tests with MSVC2012.

Reviewers: timurrrr

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5343

llvm-svn: 217763