Add a system include management interface to the toolchain, and call it

and the C++ include management routine from the proper place when
forming preprocessor options in the driver. This is the first step to
teaching the driver to manage all of the header search paths. Currently,
these methods remain just stubs in the abstract toolchain. Subsequent
patches will flesh them out with implementations for various toolchains
based on the current code in InitHeaderSearch.cpp.

llvm-svn: 143684