[clang] [Clang][SemaCXX] Fix bug where unexpanded lambda captures where assumed to have size 1 (PR #101385)

[Mital Ashok via cfe-commits]

MitalAshok wrote:

Revived from https://reviews.llvm.org/D154716

This probably needs a fresh review, I've just rebased onto main.

The problem still exists on main: https://godbolt.org/z/vPTb4qEnd

CC @cor3ntin @shafik (as people who reviewed on the phabricator)

https://github.com/llvm/llvm-project/pull/101385