ob1: OBJ_CONSTRUCT the convertor in the send_inline optimization.

This change does not appear to increase the small message latency of ping-pong
benchmarks and fixes an issue found by our ibm datatype tests.

Fixes #4232

cmr=v1.7.5:ticket=#4149
When CXX support is disabled don't check if coverage is supported. The
problem is that ompi_cxx_vendor is only defined when MPI CXX support
is enabled.