Invoke the FileChanged callback before pushing the linemarker for a system

header.  Getting it in the wrong order generated incorrect line markers in -E 
mode.  In the testcase from PR9861 we used to generate:

# 1 "test.c" 2
# 1 "./foobar.h" 1
# 0 "./foobar.h"
# 0 "./foobar.h" 3
# 2 "test.c" 2

now we properly produce:

# 1 "test.c" 2
# 1 "./foobar.h" 1
# 1 "./foobar.h" 3
# 2 "test.c" 2

This fixes PR9861.

llvm-svn: 131871