Tweak changes in r186464 to avoid a crash.

Currently, IR generation can't handle file-scope compound literals with
non-constant initializers in C++.

Fixes PR17415 (the first crash in the bug).

(We should probably change (T){1,2,3} to use the same codepath as T{1,2,3} in
C++ eventually, given that the semantics of the latter are actually defined by
the standard.)