Add a new directory path to
PlatformDarwinKernel::GetGenericSDKDirectoriesToSearch
 - /Library/Developer/KDKs where users may store
the kernel debug kits on their systems.

Change PlatformDarwinKernel::GetKextDirectoriesInSDK
to look in the root directory of places like
/Library/Developer/KDKs/KDK_10.10_14A298i.kdk
as well as the System/Library/Extensions subdir
in that directory (if it exists) and the
Library/Extensions subdir in that directory (if it
exists).
<rdar://problem/16568635>