Implement the notion of umbrella directories, which implicity cover

all of the headers below that particular directory. Use umbrella
directories as a clean way to deal with (1) directories/frameworks
that don't have an umbrella header, but don't want to enumerate all of
their headers, and (2) PrivateHeaders, which we never want to
enumerate and want to keep separate from the main umbrella header. 

This also eliminates a little more of the "magic" for private headers,
and frameworks in general.

llvm-svn: 146235