The initial IPI implementation emits an error when an api/spi-level
module publicly imports an ipi-level module. That is too aggressive for
incremental adoption.
Wrap the diagnostic in a new diagnostic group (IPIVerification) and emit
it as a warning.
Also reword the message from "...from non-internal module 'X'" to a clearer
"...because 'X' has '-library-level api/spi'"
rdar://178560797
Sam Pyankov
4742af0081