Revert D119136 "[clang] Implement Change scope of lambda trailing-return-type" and its follow-up

This reverts commit 69dd89fd.
This reverts commit 04000c2f.

The current states breaks libstdc++ usage (https://reviews.llvm.org/D119136#3455423).
The fixup has been reverted as it caused other valid code to be disallowed.
I think we should start from the clean state by reverting all relevant commits.