[windows] Fix warning about comparing ints of different signs

This fixes a warning like this:

warning: comparison of integers of different signs:
      'std::__1::__libcpp_tls_key' (aka 'long') and 'DWORD'
      (aka 'unsigned long') [-Wsign-compare]
  if (*__key == FLS_OUT_OF_INDEXES)
      ~~~~~~ ^  ~~~~~~~~~~~~~~~~~~

Differential Revision: https://reviews.llvm.org/D49782

llvm-svn: 337946