We recently added a clang-tidy job to the CI, which generates a compilation database. We can leverage that now existing database to begin running include-what-you-use over the codebase.

This PR demonstrates using a mapping_file to indicate fixups / includes that may differ from IWYU suggestions. In this case, I've added some fixups for glibc includes that I've upstreamed changes for:

# Fixups / upstreamed changes
[
  { include: [ "<bits/termios-c_lflag.h>", private, "<termios.h>", public ] },
  { include: [ "<bits/termios-struct.h>", private, "<termios.h>", public ] },
  { include: [ "<bits/termios-tcflow.h>", private, "<termios.h>", public ] },
]

The include "fixing" commits of this PR:

• Adds missing includes.
• Swaps C headers for their C++ counterparts.
• Removes the pointless / unmaintainable //for abc, xyz comments. When using IWYU, if anyone wants to see / generate those comments, to see why something is included, it is trivial to do so (IWYU outputs them by default). i.e:

// The full include-list for compat/stdin.cpp:
#include <compat/stdin.h>
#include <poll.h>                  // for poll, pollfd, POLLIN
#include <termios.h>               // for tcgetattr, tcsetattr
#include <unistd.h>                // for isatty, STDIN_FILENO

TODO:

• Qt mapping_file. There is one in the IWYU repo, but it's for Qt 5.11. Needs testing.
• Boost mapping_file. There is one in the IWYU repo, but it's for Boost 1.75. Needs testing.

I'm not suggesting we turn this on the for entire codebase, or immediately go-nuts refactoring all includes. However I think our dependency includes are now slim enough, and our CI infrastructure in place such that we can start doing this in some capacity, and just automate away include fixups / refactorings etc.