[clang][CodeGen] Break up TargetInfo.cpp [6/8]

Make `qualifyWindowsLibrary` and `addStackProbeTargetAttributes`
protected members of `TargetCodeGenInfo`.
These are helper functions used by `getDependentLibraryOption` and
`setTargetAttributes` methods when targeting Windows. The change will
allow these functions to be reused after splitting `TargetInfo.cpp`.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D150178