Ensure that when a single backspace decomposes the last NFC character in
the app context, the remainder of the 'cluster' is preserved, matching
the implication of the CLDR keyboard specification.
This addresses the behavior in #15487 where the cached context became
out of sync with the app context after deleting an entire NFC cluster
such as ê, which caused a loop ending up with the entire context being
deleted, at which point the loop exited with a fail-safe.
Note that the LDML keyboard tests (ldml.cpp) do not currently exercise
the normalization code; this is a gap that should be addressed to ensure
that we are testing final application behavior.
Fixes: #15487
- verify marker string as it's popped off
- add a missing pragma to debuglog.h
- move UErrorCode out of ldml_processor, change normalization functions to return true on success
For: #9468
