[asan] Fix shifting compiler warnings when compiler in 64-bits

Summary:
The MSVC compiler complains about implicit conversion of 32-bits constant to
64-bit when using this shiting pattern  1 << (<64-bit expr>).

Reviewers: rnk

Subscribers: kcc, llvm-commits, wang0109, kubabrecka, chrisha

Differential Revision: http://reviews.llvm.org/D21524

llvm-svn: 273267