CMake: Various improvements

Yellow squiggly lines begone!
Done automatically on .cpp files through `run-clang-tidy`, with manual corrections to the mistakes.
If an import is directly used, but is technically unnecessary since it's recursively imported by something else, it is *not* removed.
The tool doesn't touch .h files, so I did some of them by hand while fixing errors due to old recursive imports.
Not everything is removed, but the cleanup should be substantial enough.
Because this done on Linux, code that isn't used on it is mostly untouched.
(Hopefully no open PR is depending on these imports...)

Core: Let any thread call previously host-thread-only functions

The core no longer cares which thread is the host thread.

Cleaning up Android's HostThreadLock is left for another PR, in part
because the HostThreadLock in NativeConfig.cpp still serves a purpose,
and in part to make any issues easier to bisect.

Main: Add SA_RESTART to sigaction sa_flags.

Remove Host_RefreshDSPDebuggerWindow (which hasn't done anything since
DolphinWX was removed in 44b22c90) and DSP::Host::UpdateDebugger (which
only called Host_RefreshDSPDebuggerWindow).