cmake doesn't set a `-ffile-prefix-map` for source files. Among other things,
this results in absolute paths in Scylla logs:
```
Jan 11 09:59:11.462214 longevity-tls-50gb-3d-master-db-node-2dcd4a4a-5 scylla[16339]: scylla: /jenkins/workspace/scylla-master/next/scylla/utils/refcounted.hh:23: utils::refcounted::~refcounted(): Assertion `_count == 0' failed.
```
And it results in absolute paths in gdb, which makes it a hassle to get gdb to display
source code during debugging. (A build-specific `substitute-path` has to be
configured for that).
There is a `-file-prefix-map` rule for `CMAKE_BINARY_DIR`,
but it's wrong.
Patch dbb056f4f7, which added it,
was misguided.
What we want is to strip the leading components of paths up to
the repository directory, both in __FILE__ macros and in debug info.
For example, we want to convert /home/michal/scylla/replica/table.cc to
replica/table.cc or ./replica/table.cc, both in Scylla logs and in gdb.
What the current rule does is it maps `/home/michal/scylla/build` to `.`,
which is wrong: it doesn't do anything about the paths outside of `build`,
which are the ones we actually care about.
This patch fixes the problem.
Closesscylladb/scylladb#22311
16b3352ae7