Revert r372788 "Host: use the platform identifiers from LLVM (NFC)"

> Use symbolic constants for the platform identifiers rather than replicating them
> locally.

This broke the build of LLDB on Windows, see
http://lab.llvm.org:8011/builders/lldb-x64-windows-ninja/builds/9182 which
fails with e.g.

  E:\build_slave\lldb-x64-windows-ninja\llvm\include\llvm/BinaryFormat/COFF.h(96): error C2059: syntax error: 'constant'
  E:\build_slave\lldb-x64-windows-ninja\llvm\include\llvm/BinaryFormat/COFF.h(96): error C3805: 'constant': unexpected token, expected either '}' or a ','
  E:\build_slave\lldb-x64-windows-ninja\llvm\include\llvm/BinaryFormat/COFF.h(128): error C2059: syntax error: 'constant'
  ...

llvm-svn: 372847