[Backtracing][Linux] Add Linux crash handler to the runtime.

This also adds a function to demangle a symbol, and a way for the
backtracing code to report warning messages to the same place as
the main runtime.

I'd like to rename the _swift_isThunkFunction() SPI also, but we
can't do that until we've made the changes to the _Backtracing
library, so we'll do that there instead.

rdar://110261430

stdlib/public/runtime/CrashHandlerMacOS.cpp

@@ -57,7 +57,7 @@ void suspend_other_threads();
 void resume_other_threads();
 bool run_backtracer(void);
 
-swift::CrashInfo crashInfo;
+CrashInfo crashInfo;
 
 os_unfair_lock crashLock = OS_UNFAIR_LOCK_INIT;