When we create a lambda, we would skip over declaration contexts
representing a require expression body, which would lead to wrong
lookup.
Note that I wasn't able to establish why the code
in `Sema::createLambdaClosureType` was there to begin with (it's not
exactly recent)
The changes to mangling only ensure the status quo is preserved and do
not attempt to address the known issues of
mangling lambdas in require clauses.
In particular the itanium mangling is consistent with Clang before this
patch but differs from GCC's.
Fixes#147650
The previous approach broke code generation for the MS ABI due to an
unintended code path during constraint substitution. This time we
address the issue by inspecting the evaluation contexts and thereby
avoiding that code path.
This reapplies 96eced624 (#102857).
