[NFC] Rename hlsl semantics to hlsl annotations (#89309)

The attribute name "HLSLSemantics" is confusing, because semantics
aren't always the annotation that are applied to specific variables. The
name for this attribute needs to be less specific. This PR changes the
attribute name from HLSLSemantic to HLSLAnnotation, and changes the
associated function and variable names to support this conceptual
change.
The HLSLAnnotation attribute will never be output in ast-dump due to it
being parsed for the attribute that it represents. There is no
functional change, so there are no accompanying tests.