On some linuxes, /usr/include belongs to GCC and the standard
libraries that work with clang are in /usr/lib/clang/8.0.0 or
some variation thereof.
This results in errors such as:
```
/../lib64/gcc/x86_64-pc-linux-gnu/8.3.0/../../../../include/c++/8.3.0/bits/cxxabi_init_exception.h:38:10: fatal error: 'stddef.h' file not found
```
during extraction.
When processing many files that contain top-level items with the same
name (e.g. "operator<<"), the output was non-deterministic and depended
on the order in which the different Clang processes finished. This
commit adds sorting that also accounts for the filename to prevent
random changes from run to run.
