Fix raw_fd_ostream::write_impl hang due to an infinite loop with large output

On windows when raw_fd_ostream::write_impl calls write, a 32 bit input is required for character count. As a variable with size_t is used for this argument, on x64 integral demotion occurs. In the case of large files an infinite loop follows. 
See: https://bugs.llvm.org/show_bug.cgi?id=37926
This fix allows the output of files larger than the previous int32 limit.

Differential Revision: https://reviews.llvm.org/D48948

llvm-svn: 339027