The compile time improvements from using combined TUs mostly isn't
worth the annoyance they cause with various IDE shortcuts, like
when switching between header and its impl. file.
Splitting them apart also fixes the issue of empty subdirs being
installed due to `foo/internal` folders that only contained the
combined TUs and no headers.
Closes#2457Closes#2463
This should improve the compilation times by decreasing the number
of TUs compiled, without making overly big TUs that would cause
problems with heavy-tailed compilation times.
There is one "combined TU" for the top level part, and each subpart,
except for Reporters, which currently do not have any trivial TUs.
