Triple: Make setEnvironment not override the object format

Discovered by Halide users who had C++ code like this:
  Triple.setArch(Triple::x86);
  Triple.setOS(Triple::Windows);
  Triple.setObjectFormat(Triple::ELF);
  Triple.setEnvironment(Triple::MSVC);

This would produce the stringified triple of x86-windows-msvc, instead
of the x86-windows-msvc-elf string needed to run MCJIT.

With this change, they retain the -elf suffix.

llvm-svn: 229160