[clang][RecoveryExpr] Clarify the dependence-bits documentation.

The expr dependent-bits described that the expression somehow
depends on a template paramter.

With RecoveryExpr, we have generalized it to "the expression depends on
a template parameter, or an error".  This patch updates/cleanups all related
comments of dependence-bits.

Differential Revision: https://reviews.llvm.org/D83213