[Clang] Do not check for underscores in isAllowedInitiallyIDChar

isAllowedInitiallyIDChar is only used with non-ASCII codepoints,
which are handled by isAsciiIdentifierStart.
To make that clearer, remove the check for _ from
isAllowedInitiallyIDChar, and assert on ASCII - to ensure neither
_ or $ are passed to this function.

Reviewed By: tahonermann, aaron.ballman

Differential Revision: https://reviews.llvm.org/D130750