Merge commit f1b0a4fc540f from llvm git (by Richard Smith):
An expression should only contain an unexpanded parameter pack if it
lexically contains a mention of the pack.
Systematically distinguish between syntactic and semantic references to
packs, especially when propagating dependence from a type into an
expression. We should consult the type-as-written when computing
syntactic dependence and should consult the semantic type when computing
semantic dependence.
Fixes#54402.
MFC after: 3 days
3b7f365e4d