Simplify typename_concat

This patch simplifies typename_concat, changing the return type and
removing the obstack allocation code.  The latter is possible because
the only caller using this mode uses the name when creating a new
type, and 'new_type' copies the string to the appropriate obstack
anyway.  It also changes typename_concat to use 'concat'.  This change
lets us remove a mildly fragile macro as well.

Approved-By: Simon Marchi <simon.marchi@efficios.com>