Use a consistent spelling for vtables.

include/clang/Basic/TargetCXXABI.h

lib/CodeGen/CGClass.cpp

lib/CodeGen/CGObjCMac.cpp

lib/CodeGen/CGVTables.cpp

lib/CodeGen/CodeGenModule.cpp

lib/CodeGen/ItaniumCXXABI.cpp

lib/CodeGen/MicrosoftCXXABI.cpp

test/CodeGenCXX/vtable-key-function-arm.cpp

test/CodeGenCXX/vtable-key-function-ios.cpp