The only remaining uses of build_vector are in the selftests in tree.c.
This patch makes it static and moves it to the selftest part of the file.

2017-12-07  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* tree.c (build_vector): Delete.
	* tree.h (build_vector): Make static and move into the self-testing
	block.

From-SVN: r255479