Revert 259942, r259943, r259948.

The Windows bots have been failing for the last two days, with:

FAILED: C:\PROGRA~2\MICROS~1.0\VC\bin\amd64\cl.exe -c LLVMContextImpl.cpp
D:\buildslave\clang-x64-ninja-win7\llvm\lib\IR\LLVMContextImpl.cpp(137) :
    error C2248: 'llvm::TrailingObjects<llvm::AttributeSetImpl,
                                        llvm::IndexAttrPair>::operator delete' :
        cannot access private member declared in class 'llvm::AttributeSetImpl'
    TrailingObjects.h(298) : see declaration of
        'llvm::TrailingObjects<llvm::AttributeSetImpl,
                               llvm::IndexAttrPair>::operator delete'
    AttributeImpl.h(213) : see declaration of 'llvm::AttributeSetImpl'

llvm-svn: 260053