[Clang] Fix references to captured variables in dependant context.

D119136 changed how captures are handled in a lambda call operator
declaration, but did not properly handled dependant context,
which led to crash when refering to init-captures in
a trailing return type.

We fix that bug by making transformations more symetric with parsing,
ie. we first create the call operator, then transform the capture,
then compute the type of the lambda call operaror.

This ensures captures exist and have the right type when
we parse a trailing requires-clause / return type.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D124012