Fix an assertion-on-error during tentative constructor parsing by

propagating error conditions out of the various annotate-me-a-snowflake
routines.  Generally (but not universally) removes redundant diagnostics
as well as, you know, not crashing on bad code.  On the other hand,
I have just signed myself up to fix fiddly parser errors for the next
week.  Again.

llvm-svn: 97221