OpenMP RTL cleanup: eliminated warnings with -Wcast-qual.

Changes are: replaced C-style casts with cons_cast and reinterpret_cast;
type of several counters changed to signed; type of parameters of 32-bit and
64-bit AND and OR intrinsics changes to unsigned; changed files formatted
using clang-format version 3.8.1.

Differential Revision: https://reviews.llvm.org/D34759