[clang][NFC] Use "notable" for "interesting" identifiers in `IdentifierInfo` (#81542)

This patch expands notion of "interesting" in `IdentifierInto` it to
also cover ObjC keywords and builtins, which matches notion of
"interesting" in serialization layer. What was previously "interesting"
in `IdentifierInto` is now called "notable".

Beyond clearing confusion between serialization and the rest of the
compiler, it also resolved a naming problem: ObjC keywords, notable
identifiers, and builtin IDs are all stored in the same bit-field. Now
we can use "interesting" to name it and its corresponding type, instead
of `ObjCKeywordOrInterestingOrBuiltin` abomination.