Repair recognition of "CC" as an alias for the C++ compiler, since it
was silently broken by upstream for a Windows-specific use-case.

Apparently some versions of CMake still rely on this archaic feature...

Reported by:	rakuco
MFC after:	3 days
X-MFC-With:	r261991
Garbage collect the old way of adding the libstdc++ include directories
in clang's InitHeaderSearch.cpp.  This has been superseded by David
Chisnall's commit in r255321.

Moreover, if libc++ is used, the libstdc++ include directories should
not be in the search path at all.  These directories are now only used
if you pass -stdlib=libstdc++.

MFC after:	3 days
X-MFC-With:	r261991
Enhance the mechanism that lets you configure the ubldr boot device by
setting the u-boot environment variable loaderdev=.  It used to accept only
'disk' or 'net'.  Now it allows specification of unit, slice, and partition
as well.  In addition to the generic 'disk' it also accepts specific
storage device types such as 'mmc' or 'sata'.

If there isn't a loaderdev env var, the historical behavior is maintained.
It will use the first storage device it finds, or a network device if
no working storage device exists.

99% of the work on this was done by Patrick Kelsey, but I made some
changes, so if anything goes wrong, blame me.

Submitted by:	Patrick Kelsey <kelsey@ieee.org>
Don't try to return a vector to a caller that only cares if a vector
is pending or not.

Approved by:	neel (co-mentor)