Revert r218925 - "Patch to warn if 'override' is missing"

This CL has caused bootstrap failures on Linux and OSX buildbots running with -Werror.

Example report from http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux/builds/13183/steps/bootstrap%20clang/logs/stdio:

================================================================
[ 91%] Building CXX object tools/clang/tools/diagtool/CMakeFiles/diagtool.dir/ShowEnabledWarnings.cpp.o
In file included from /home/dtoolsbot/build/sanitizer-x86_64-linux/build/llvm/lib/Target/R600/AMDGPUISelDAGToDAG.cpp:20:
In file included from /home/dtoolsbot/build/sanitizer-x86_64-linux/build/llvm/lib/Target/R600/SIISelLowering.h:19:
/home/dtoolsbot/build/sanitizer-x86_64-linux/build/llvm/lib/Target/R600/SIInstrInfo.h:71:8: error: 'getLdStBaseRegImmOfs' overrides a member function but is not marked 'override' [-Werror,-Winconsistent-missing-override]
  bool getLdStBaseRegImmOfs(MachineInstr *LdSt,
       ^
/home/dtoolsbot/build/sanitizer-x86_64-linux/build/llvm/include/llvm/Target/TargetInstrInfo.h:815:16: note: overridden virtual function is here
  virtual bool getLdStBaseRegImmOfs(MachineInstr *LdSt,
               ^
================================================================