Switch to a C-style cast here to silence a brain-dead MSVC warning. It

complains about the truncation of a 64-bit constant to a 32-bit value
when size_t is 32-bits wide, but *only with static_cast*!!! The exact
signal that should *silence* such a warning, and in fact does silence it
with both GCC and Clang.

Anyways, this was causing grief for all the MSVC builds, so pointless
change made. Thanks to Nikola on IRC for confirming that this works.

llvm-svn: 152021