Fix MSVC warning.

This patch fixes the following MSVC warning.

  warning C4334: '<<' : result of 32-bit shift implicitly
  converted to 64 bits (was 64-bit shift intended?)

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@222293 91177308-0d34-0410-b5e6-96231b3b80d8