PR35815: Separate out the ns-consumed diagnostic into an error and

a warning

This commit separates out the warn_nsconsumed_attribute_mismatch and
warn_nsreturns_retained_attribute_mismatch diagnostic into a warning and error.
This is needed to avoid a module import regression introduced by r313717 that
turned these errors into warnings and started promoting them only when needed,
which caused an error when importing a module as it had different warning
settings.

rdar://36265651

llvm-svn: 321775