avoid emitting a bogus line marker for the top level #include

location.  We now -E a file containing "foo" into:

# 1 "t.c"
# 1 "t.c" 1
# 1 "<predefines>" 1
# 1 "t.c" 2
foo

instead of:

# 1 "t.c"
# 1 "t.c" 1
# 0 "t.c"
# 1 "<predefines>" 1
# 1 "t.c" 2
foo

llvm-svn: 63405