emulate a bit of GCC path lookup weirdness: if a system

directory is shadowed by a user directory in the lookup
path, ignore the user directory not the system one.  Not
doing this can affect file lookup and the "is a system
header" bit on locations.  For example:
  clang -v -I/usr/include inc.c -E | & grep /usr/inc
now prints:

# 1 "/usr/include/i386/_types.h" 1 3 4
# 37 "/usr/include/i386/_types.h" 3 4
# 70 "/usr/include/i386/_types.h" 3 4

instead of:

# 1 "/usr/include/i386/_types.h" 1
# 37 "/usr/include/i386/_types.h"
# 70 "/usr/include/i386/_types.h"

This is part of rdar://6243860.

llvm-svn: 56669