Bug 23876 – segfault / ICE regression with packages

reproduction test case: reduced.d ```d void onInvalidType() { import mystd.meta; import mystd.range; } ``` mystd/meta.d ```d import mystd.range.primitives ; ``` mystd/range/package.d ```d import mystd.range; ``` mystd/range/primitives.d ```d // file is empty ``` dmd -c reduced.d

smaller reduced.d: ```d import mystd.meta; import mystd.range; ```

This seems to be a gc bug caused by the compiler implementation code. When the compiler parses the import declarations, to be more precise, the chain of packages separated by dots it needs to concatenate the identifiers to an array. That concatenation calls the _d_arrayappend hook, which currently is not templated which calls some conservative gc methods. The failure is somewhere in the innards of those functions, but up until this point I haven't been able to precisely pinpoint it. Changing to druntime bug.

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