Sink the strange '-stdlib=...' flag handling into the C++ include

Sink the strange '-stdlib=...' flag handling into the C++ include
handling logic of the generic ToolChain. This flag, despite its name,
has *nothing* to do with the GCC flag '-nostdlib' that relates
(exclusively) to the linking behavior. It is a most unfortunate name in
that regard...

It is used to tell InitHeaderSearch.cpp *which* set of C++ standard
library header search paths to use -- those for libstdc++ from GCC's
installation, or those from a libc++ installation. As this logic is
hoisted out of the Frontend, and into the Driver as part of this
ToolChain, the generic method will be overridden for the platform, where
it can implement this logic directly. As such, hiding the CC1 option
passing in the generic space is a natural fit despite the odd naming.

Also, expand on the comments to clarify whats going on, and tidy up the
Tools.cpp code now that its simpler.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@143687 91177308-0d34-0410-b5e6-96231b3b80d8