Reformat.
VirtualFileSystemTest.cpp: Get rid of initializer list on std::vector, to appease msc17.
Tools.cpp: Update getLinuxDynamicLinker() to return Twine instead of StringRef, since r211598 has introduced manipulation of return string.
MCNullStreamer.cpp: Roll back a few empty methods that have been marked as unreachable in MCStreamer.cpp.

  void EmitCOFFSecRel32(MCSymbol const *Symbol) override {}
  void EmitGPRel32Value(const MCExpr *Value) override {}

It should fix crash like "llc -mtriple=i686-cygwin -filetype=null".