[Clang] Fix evaluation of parameters of lambda call operator attributes

Fix a regresion introduced by D124351.
Attributes of lambda call operator were evaluated in the
context of the closure object type rather than its operator,
causing an assertion failure.

This was because we temporarily switch to the class lambda to
produce the mangling of the lambda, but we stayed in that
context too long.

Reviewed By: eandrews, aaron.ballman

Differential Revision: https://reviews.llvm.org/D146535