flushMixins

we want to write the mixin expansion file also on error, but there are too many ways to terminate dmd (e.g. fatal() which calls exit(EXIT_FAILURE)), so we can't rely on scope(exit) ... in tryMain() actually being executed so we add atexit(&flushMixins); for those fatal exits (with the GC still valid)

extern (C)
void
flushMixins
()

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