Add QUIC_EXPORT_PRIVATE macros to qpack_index_conversions.h functions.
Add QUIC_EXPORT_PRIVATE macros to qpack_index_conversions.h functions.

This is to unblock QUICHE roll into Chromium, which currently fails to link.

Also remove stale comments (I meant to delete them before submitting
change 263421099.)

gfe-relnote: n/a.  QUIC_EXPORT_PRIVATE is a no-op in Google3.
PiperOrigin-RevId: 263748009
Change-Id: I88efb758f14357c45f72434337bfae3950058524

google/quiche,google/quiche,google/quiche,google/quiche