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.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@97221 91177308-0d34-0410-b5e6-96231b3b80d8