[D] Remove unchecked to_constant in VECTOR_TYPE handling

The SVE port now tries to register variable-length VECTOR_TYPEs
at start-up, so it's no longer possible to use the asserting
to_constant on the number of vector elements.  This patch punts
on variable element counts instead, just like we do for other
things that the frontend doesn't recognise.

The brace indentation matches the surrounding style.

2019-11-04  Richard Sandiford  <richard.sandiford@arm.com>

gcc/d/
	* d-builtins.cc (build_frontend_type): Cope with variable
	TYPE_VECTOR_SUBPARTS.

From-SVN: r277793