8061549: Disallow _ as a one-character identifier
Summary: Underscore is no longer a one-charater identifier with -source 9
Reviewed-by: mcimadamore, jjg

8007427: Annotation element as '_' gives compiler error instead of a warning
8007401: Write test to check for generation of warnings when '_' is used as an identifier
Summary: Extended identifier production not used in annotation values
Reviewed-by: jjg
Contributed-by: sonali.goel@oracle.com