Genericize the existing logic for removing duplicate header dirs to apply

the "system dirs win over user dirs" logic to framework and headermap
search locations as well as normal directories.  This means that
clang t.m -F/System/Library/Frameworks  will treat /System/Library/Frameworks
as a system directory not a user directory.  If you use -v, the difference is:

Before:
ignoring nonexistent directory "/usr/libdata/gcc41"
ignoring duplicate framework "/System/Library/Frameworks"
#include "..." search starts here:
#include <...> search starts here:

After:
ignoring nonexistent directory "/usr/libdata/gcc41"
ignoring duplicate directory "/System/Library/Frameworks"
  as it is a non-system directory that duplicates a system directory
#include "..." search starts here:
#include <...> search starts here:

This fixes rdar://6566429.

llvm-svn: 64060