[PR29906] Handle DW_AT_import <base type>

[Alexandre Oliva]

We used to ICE with the testcase in the patch below because
force_type_die would not create dies for base types.

In most cases, it doesn't have to, since it's used for contexts of
other types or declarations, and these are never base types (right?)

But in the one case adjusted by the patch below, namely using
declarations that create aliases for types, it can be a base type.

Instead of burdening all uses with an additional unneeded test in
force_type_die itself, I figured it made more sense to handle this
special need in the only (provided that my assumption above holds)
location where this case makes sense.

Bootstrapping and regtesting on x86_64-linux-gnu, but this looks
obvious enough to me that I thought I'd go ahead and post it.

Ok to install?

:ADDPATCH debug:

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-- 
Alexandre Oliva         http://www.lsd.ic.unicamp.br/~oliva/
FSF Latin America Board Member         http://www.fsfla.org/
Red Hat Compiler Engineer   aoliva@{redhat.com, gcc.gnu.org}
Free Software Evangelist  oliva@{lsd.ic.unicamp.br, gnu.org}