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

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

This reverts commit 69dd89fdcbd846375a45e2fe3a88710887236d7a.
This reverts commit 04000c2f928a7adc32138a664d167f01b642bef3.

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.