Don't mutate annotation metadata when merging attrs

Prior to this commit, invoking the getMergedAnnotationAttributes()
method in AnnotationReadingVisitorUtils resulted in mutation of the
internal state of the ASM-based annotation metadata supplied to the
method.

This commit fixes this issue by making a copy of the original
AnnotationAttributes for the target annotation before merging attribute
values from the meta-annotation hierarchy.

This commit also introduces a slight performance improvement by
avoiding duplicate processing of the attributes of the target
annotation.

Issue: SPR-11710