fix: sccache does not generate new object file if "-gsplit-dwarf" is enabled and the filename or directory of the object is changed

[zhouronghua]

when output dir changed, and -g -gsplit-dwarf is enabled, new object file should be gererated, but not old cached object.

case:

1 echo "int test(){}" > test.cc

2 mkdir o1 o2

3 sccache g++ -c -g -gsplit-dwarf test.cc -o o1/test.o

4 sccache -z

5 sccache g++ -c -g -gsplit-dwarf test.cc -o o2/test.o

6 strings o2/test.o |grep o2/test.dwo